Preheat oven to 350°F and lightly spray a 9×13" baking dish.
Heat vegetable oil in a large skillet over medium; add crumbled sausage and cook until browned, about 6–7 minutes.
1 Tbsp vegetable oil
Stir in diced onion and green pepper; sauté until softened and fragrant, about 3–4 minutes.
1 lb breakfast sausage, 0.5 cup onion
Whisk eggs, milk, salt, and pepper in a bowl; pour into skillet and gently scramble until just set.
0.5 cup green bell pepper, 8 large eggs, 0.25 cup milk, 1 tsp salt
Stir in 1 cup shredded cheddar until melted and glossy; remove from heat.
0.25 tsp black pepper
Assembly
Pour half of the enchilada sauce into the prepared dish and spread evenly.
Divide the egg–sausage filling among warmed tortillas, roll tightly, and place seam-side down in the dish.
1 cup cheddar cheese
Top enchiladas with remaining sauce and sprinkle with 1 cup cheese.
1 can (10 oz) red enchilada sauce
Bake for 20–25 minutes, until sauce bubbles and cheese is lightly golden.
Let rest 5 minutes before serving.
Notes
During testing, warming each tortilla in a dry skillet for about 10 seconds made rolling easier and prevented tearing.Use clean utensils when storing leftovers and refrigerate within two hours of cooking to maintain freshness and safety.For make-ahead convenience, assemble enchiladas, cover tightly, and refrigerate for up to 24 hours before baking as directed.