Preheat oven to 350°F. Grease (1) 9×13-inch baking dish. Set aside.: When the oven reaches an even temperature, you will notice a consistent ambient warmth and a gentle heat haze near the racks. I preheat thoroughly so the casserole begins to set immediately, giving a tender but structured center rather than a slow, uneven rise. If you skip full preheating the edges might overcook before the center firms. A common mistake is opening the oven door frequently, which drops temperature and prolongs baking.
Cook the breakfast sausage in a skillet over medium-high heat until cooked through and browned. If needed and desired, drain the grease. Set aside.: The greased surface prevents sticking and encourages even browning along the edges. Brush a thin, even layer of oil or butter in every corner and across the base so the biscuit bottom releases cleanly. If the dish is not greased well, the base can cling and tear when serving. Avoid excess grease pooling, which leads to soggy spots.
Arrange the biscuits in the bottom of the greased baking dish. Use your hands to press the biscuits flat to create an even layer across the bottom of the pan.: As the breakfast sausage hits the hot skillet you will hear a steady sizzle and smell savory, toasty notes. Cook until the meat loses pink, and you see golden browned bits that add deep flavor. Draining excess grease concentrates the texture and prevents a greasy bake. Underbrowned sausage will deliver less flavor, while overcooking until very dry can make the texture crumbly.
Evenly top the biscuits with the cooked sausage, diced bell pepper, and sliced scallions. Top evenly with the shredded cheese.: Pressing the biscuits flat creates a uniform base that soaks up custard and browns nicely. Use a gentle, even pressure so the layer is level across the pan, because an uneven base causes pockets of underbaked dough. If the biscuits remain too thick they may not cook through; too thin and they become brittle.
In a bowl or large measuring cup, whisk together the eggs, milk, salt, and several grinds of black pepper. Evenly pour over the other ingredients in the baking dish.: Layering distributes flavors and textures, so aim for an even scatter so each slice has balanced elements. The diced bell pepper should look bright, not wilted, and the scallions remain visible as green threads. Crowding one area with too much filling leads to dense spots that alter baking time.
Bake for 40-50 minutes, or until the center is puffed and the eggs are set.: As the cheddar cheese melts it will create adhesive pockets that hold layers together and brown on top. Sprinkle uniformly so you get consistent melting and little golden bubbles across the surface. Dumping cheese in one heap results in uneven melting and texture differences.
Whisk together the eggs milk salt and black pepper: The whisking should produce an even, pale yellow liquid with a few gentle air bubbles. This uniformity ensures the custard sets consistently and avoids streaks of dense white. If you under whisk, the texture can be patchy; over whisking will incorporate too much air, which can cause excessive puffing and collapse.
Evenly pour over the other ingredients: Pour slowly and move the dish gently if needed so the egg mixture distributes into crevices and around fillings. You want the custard to seep into the biscuit base and fill voids without washing away toppings. Pouring too quickly can displace toppings and create uneven pockets.
Bake for 40 to 50 minutes: As the casserole bakes you will notice the center rising slightly and the edges turning golden brown. When the center looks set and no longer jiggles, and a knife inserted near the center comes out mostly clean, it is done. Baking too long will dry out the custard and make the texture rubbery; too short and the center will be runny. Keep an eye on the oven during the last stretch so the top reaches a pleasant golden color without burning.
