Preheat oven to 375°F and lightly grease a 9×13 baking dish with nonstick cooking spray.: The oven warming creates an even baking environment, and greasing the 9 x 13 Casserole Dish prevents sticking. You should notice the oven reach an even heat and the dish feel room temperature before assembly. This step ensures the top browns evenly and helps the casserole release cleanly. If you skip greasing, the edges can cling and tear when serving, so always prep the dish first. Smell the faint warm air from your oven when it hits target temperature, and give it a few extra minutes for stability.
In a large skillet, add sausage, onion, and red bell pepper. Cook over medium-high heat, stirring frequently, until sausage is browned and no longer pink. Drain grease if needed. Stir in salt, garlic powder, paprika, and black pepper.: As the ground breakfast sausage sizzles, it will release aromatic fats and caramelize, creating deep browned bits on the pan bottom. The diced onion will soften and sweeten, while the red bell pepper becomes tender and fragrant. Stirring frequently promotes even browning and prevents burning. Drain excess grease if the pan becomes too oily, because too much fat can make the casserole greasy and impact the sauce texture. You will hear a steady sizzle and smell roasted meat and sweet onion, which are signs you are on track.
Add cooked rotini pasta to the prepared baking dish. Spread sausage mixture evenly over the pasta.: The rotini pasta should be warm and slightly firm; adding the hot sausage mixture distributes heat and prevents the pasta from clumping. Spread the browned meat and softened vegetables evenly so each forkful will have balanced flavor. Press gently so the layers settle, but avoid compacting tightly, which can lead to a dense bake. If the pasta seems too soft, it may have been overcooked earlier, so keep a firmer texture when boiling.
In the same skillet, melt butter. Add flour and whisk constantly for 1 minute to form a roux. Slowly whisk in milk, a little at a time, until smooth. Bring mixture to a gentle boil, then reduce heat and simmer 2–3 minutes until thickened. Remove from heat and stir in 1 cup of shredded cheese until melted.: As the butter melts and foams, whisking in the all purpose flour forms the roux, which should smell nutty but not toasted. Whisk while adding milk gradually to maintain a glossy, lump free texture. When the sauce reaches a gentle boil, it will thicken and coat the back of a spoon, indicating readiness. Remove from heat before adding the first cup of Colby Jack cheese so it melts smoothly into the sauce without breaking. Lumps form when liquid is added too quickly, so patience here yields a silky finish and a sauce that clings perfectly to pasta and sausage.
Sprinkle 1 cup of cheese over the sausage and pasta in the baking dish. Pour cream sauce on top and stir gently so everything is coated.: When you add the first cup of shredded Colby Jack cheese across the pasta and sausage, it creates pockets of melted goodness that mingle with the sauce. Pour the warm cream sauce over evenly and stir gently to coat, watching for an even sheen across the pasta. You want the sauce to nestle into the rotini pasta grooves and around sausage pieces. Avoid over stirring which can crush the texture of the pasta and make the dish dense rather than airy.
In a medium bowl, whisk eggs. Stir in the remaining 1 cup of cheese. Add biscuit pieces and toss to coat. Spread biscuit mixture evenly over the pasta and sauce.: The whisked large eggs blended with another cup of Colby Jack cheese and torn biscuits form a binder that will set into a golden, slightly custardy topping. Tossing ensures each biscuit piece is coated and will absorb some sauce as it bakes. Spread the mixture uniformly so the top browns evenly in the oven. If pieces are uneven or clumped, some will remain dense while others crisp, so aim for an even layer with consistent piece sizes for uniform texture and appearance.
Place dish in the oven and bake for 25 minutes, or until a knife inserted in the center comes out clean and the middle is hot. Let casserole rest for 5 minutes before serving warm.: During baking you should see bubbling edges and a golden biscuit topping, with a fragrant aroma of melted Colby Jack cheese and roasted sausage. The center temperature rising and a clean knife test confirm the custard like binder has set. Resting for 5 minutes lets juices redistribute and makes slicing easier, while slightly cooling the surface so it holds shape. A common error is cutting too soon, which causes the layers to run; patience here yields neat serving portions and a balanced texture that is both saucy and structured.
