Preheat oven to 400 F/200 C.: As the oven warms you will notice a steady, dry heat that is essential for roasting instead of steaming. The hot environment encourages Maillard reactions on the surface of the butternut squash , producing nutty browning and a sweet smell that deepens as roasting continues. A common mistake is not fully preheating, which leads to uneven color and longer cook times. Make sure the rack is placed to allow air to circulate around the pan so the cubes crisp at the edges.
Mix together the olive oil, 1 T balsamic vinegar, and rosemary, then toss squash cubes with the mixture.: Right after combining these, breathe in the fresh herb aroma and the faint fruity note from the vinegar, which meld with the olive oil into a glossy coating. Coating each cube ensures even browning and prevents sticking, while the small amount of vinegar deepens flavor without preventing caramelization. If you crowd the pan when tossing, the pieces will steam rather than roast, so use a bowl spacious enough to toss freely.
Arrange the squash on a cookie sheet or dish large enough to hold it in a single layer.: Laying the butternut squash in one layer is crucial for direct contact with the hot surface, which creates the golden edges you want to see. As you spread the cubes, look for even spacing; they should not touch too closely. Overlapping pieces will stay pale and soft rather than gaining that roasted texture, so use two pans if necessary to keep space between cubes.
Roast 20 minutes, then turn squash and roast about 20 minutes more, or until squash is very soft and starting to get slightly caramelized.: During the first stint in the oven you will see the edges begin to color and the scent deepen, and after turning the cubes you should notice more pronounced caramelization. The flesh should be tender when pierced and the surfaces should show golden patches. A typical slip up is opening the oven too frequently, which can lower the temperature and extend cooking time; rely on visual cues and a fork test near the end.
Remove from oven and toss squash with additional 1 tsp balsamic vinegar, sea salt, and pepper. You can mash slightly if you want, but I keep mine rather chunky. Serve hot.: Once out of the oven, the hot butternut squash will eagerly absorb the finishing vinegar, which brightens the rich roasted sweetness. Tossing while still hot ensures the flavors meld quickly and the vinegar mellows into a glossy finish. Taste as you season with sea salt and black pepper , because oven cooking can concentrate flavors differently than you expect. A common oversight is adding too much vinegar at the start, leaving nothing to enliven the finished dish.
This is also excellent made ahead and reheated. For a holiday like Thanksgiving where there are lots of demands on the oven, it could be cooked early in the day and kept warm in a crockpot while other things bake.: When reheating you will get a faint reprise of roasted aroma and the texture may soften a bit more, which is fine for make ahead service. I often reheat gently in a low oven or a warm slow cooker to avoid drying the cubes, and stirring occasionally restores an even texture. Avoid high heat reheats which can overbrown or dry the pieces; aim for gentle warmth so the squash remains luscious.
