Brown the Sausage: Cook breakfast sausage in a large skillet over medium heat until fully browned and no pink remains, breaking it up into crumbles as it cooks. Once cooked, remove half of the sausage and set aside for later use in the casserole.
Prepare the Gravy: Add butter to the remaining sausage in the skillet and allow it to melt. Sprinkle the flour over the sausage and butter mixture, stirring constantly to incorporate. Continue cooking for about 3 minutes to eliminate the raw flour taste and create a roux.
Create a Smooth Sauce: Gradually whisk in the half-and-half, adding it slowly to prevent lumps from forming. Continue stirring until the mixture thickens into a smooth gravy. Season with Cajun seasoning (or salt), black pepper and red pepper flakes, adjusting to your taste preference.
Prepare the Biscuits: Cut each biscuit into nine pieces to create small, bite-sized portions that will cook evenly and absorb the flavors of the casserole.
Mix Egg Mixture: In a separate bowl, whisk together eggs, milk, and 1/4 teaspoon of salt or Cajun seasoning until well combined.
Layer the Casserole: Grease a 9×13 baking dish. Layer the cut biscuit pieces on the bottom of the dish, followed by the reserved sausage, and 2 cups of the mixed cheeses.
Add Wet Ingredients: Pour the egg mixture evenly over the layered ingredients, ensuring the biscuits are well covered. Then, spoon the prepared sausage gravy over the top and finish with the remaining cheese.
Bake to Perfection: Place the casserole in a preheated 350°F oven and bake uncovered for 35-40 minutes, or until the casserole is set in the center and the top is golden brown and bubbly.