Preheat oven to 400 degrees F. In a large mixing bowl, toss the butternut squash cubes with the olive oil and garlic till the squash is evenly coated with the oil.: The kitchen fills with a slight warmth even before the squash goes in, and that preheated oven helps the butternut squash develop caramelized edges rather than steaming. You'll notice a toasted, sweet scent as the surface browns. If your oven runs hot, it can char the squash, so use an oven thermometer to avoid burning.
Spread the squash out on a baking sheet in an even layer. You can line the sheet with aluminum for easier cleanup, if you wish. Sprinkle the squash lightly with salt. Place squash in the oven and let it roast for about 35 minutes, turning once during cooking, till squash is tender but not overly soft.: When you toss the pieces, the glossy sheen from the olive oil signals they've been coated and will brown evenly. The crushed garlic will cling to the cubes, slowly infusing them as they roast. If cubes are uneven, some will overcook while others remain firm, so aim for uniform size to prevent a mix of underdone and mushy pieces.
Meanwhile, spread the chopped walnuts in an even layer in a small skillet. Toast the walnuts over medium heat, stirring frequently, till they're toasted and fragrant. Be careful not to let the walnuts burn-- as soon as they're toasted, pour them out of the skillet to keep them from browning further.: An even single layer promotes even roasting and encourages crisping on the exposed faces. The light salt helps draw out moisture and amplify sweetness. Overcrowding traps steam and leads to soggy results, so use two pans if necessary to keep space between cubes.
In a medium saucepan, melt butter over medium heat. Whisk in the flour, salt, and pepper. Stir to form a thick paste.: As the butternut squash roasts, listen for the oven's quiet hum and smell the transformation as sugars caramelize and edges brown. Turning once halfway encourages even golden color. If you skip turning, one side may overbrown while the other stays pale, affecting both texture and flavor.
Slowly whisk in the milk, 1/4 cup at a time. Heat the milk mixture over medium heat for a few minutes, whisking frequently, till it thickens and begins to bubble around the edges. Do not let the sauce boil.: Toasting releases oils and a deep, nutty aroma that elevates the gratin. You will see the walnuts darken slightly and a fragrant scent will fill the pan. Move them constantly because walnuts can go from toasted to burnt quickly, and burnt nuts taste acrid instead of sweet.
Whisk in the grated Gruyère cheese and stir till melted. Whisk in a pinch of nutmeg and more salt to taste, if desired. Reduce heat to lowest setting, stirring frequently, until ready to assemble the gratin.: Forming this roux creates the backbone of the sauce. The mixture will become a thick paste and develop a faintly toasted scent, which removes raw flour notes. Cook just long enough to smell the flour mellow, because overcooking will darken the paste and alter the sauce color and taste.
When the squash is done roasting, remove it from the oven and turn the temperature down to 375 degrees F. Place the squash into a gratin dish or 8 x 8 inch squash baking dish in an even layer.: Adding small amounts ensures a silky emulsion without lumps. As it cooks, you will see the sauce thicken and small bubbles form at the edges, a sign it is nearing readiness. Avoid boiling, because high heat can cause the sauce to separate or scorch on the bottom.
Sprinkle the toasted walnuts evenly across the top of the squash.: When the Gruyère cheese meets the warm sauce, it should melt smoothly into a glossy, cohesive mixture, releasing a nutty aroma. The tiny pinch of nutmeg lifts the dairy notes. If the heat is too high, the cheese can seize into oily clumps, so keep the temperature low and stir frequently.
Pour the Gruyère cheese sauce evenly across the top of the butternut squash. Place the assembled gratin into the oven and bake for 20-25 minutes till edges begin to brown and the cheese sauce is bubbly.: Transferring the warm cubes into the dish helps set the structure of the gratin. An even layer ensures that every area gets sauced and baked consistently. Piling too high creates uneven baking, with the center potentially staying cooler and undercooked.
Remove the gratin from the oven and turn on your broiler. When broiler is hot, place the gratin back in the oven and let it brown under the broiler for 1-2 minutes, watching it carefully, till the top is browned to your liking. Serve hot.: Distributing the crunchy pieces across the surface guarantees contrast in each bite. The toasted scent will mingle with the roasted butternut squash , signaling readiness for assembly. Dumping them all in one spot reduces textural balance, so spread them thoughtfully.
Vegetarian note: If you are strictly vegetarian (no cheese with rennet), make sure you find a cheese with a microbial rennet.: When the glossy sauce flows over the cubes it should coat and nestle between them, which helps bind the dish and deliver creamy pockets of flavor. You can gently tilt the dish to help the sauce settle. If the sauce appears too thin, a few extra minutes on low heat to simmer and reduce will thicken it before pouring.
Bake the assembled gratin at 375 degrees F for 20 to 25 minutes until edges brown and the sauce is bubbly: As it bakes, the aroma intensifies and the surface will begin to take on a warm, golden hue. Bubbles at the edges mean the sauce is hot through. Underbaking leaves the sauce runny and the center cooler, so aim for visual cues rather than strictly the clock.
Broil the gratin for 1 to 2 minutes to brown the top, watching carefully: A quick broil creates a beautifully caramelized top and a slight crunch. It happens fast, so stand by the oven and watch closely to prevent burning. If you walk away, the broiler will darken the surface beyond pleasant, creating a bitter finish.
Serve hot: When you bring it to the table, the gratin should be bubbling and warm, with a fragrant pull of melted Gruyère cheese . The contrast between silky squash and crunchy walnuts will be at its peak. If it sits too long, the sauce firms up and the texture changes, so serve promptly for the best mouthfeel.
