In a large skillet, brown the sausage over medium-high heat, breaking it up into small pieces with a wooden spoon.
Reduce the heat to medium-low. When the sausage is cooked and no longer pink, sprinkle half of the 1/3 cup flour on the sausage and stir until it is absorbed. Add the rest a little at a time, and stir until all the flour is soaked up. Stir for 1 more minute to get the flour taste out.
Turn the heat to medium. Pour in the milk, 1 cup at a time, stirring to thicken it up each time.
Once you have added all the milk, continue stirring for another 10-12 minutes until the gravy has thickened up some.
Add 1/2 teaspoon seasoning salt and 1 and 1/2 to 2 teaspoons pepper.
Meanwhile, mix together the eggs. Break the eggs into a large bowl or stand mixer and whisk. Add 1/2 cup milk, 3/4 teaspoons seasoning salt, mustard, paprika, 1/2 teaspoon salt, and pepper to taste.*
Grease a 9x13 inch casserole dish. Open the can of biscuits and separate all the biscuits. Use scissors to cut each biscuit into 4 pieces.
Drop the quartered biscuits onto the prepared pan and spread them out evenly.
Pour the egg mixture over the biscuits.
Sprinkle with 1 cup cheese.
Pour the gravy over the cheese and spread it out.
(At this point you can cover tightly and refrigerate overnight).
Bake in a preheated 350 degrees F oven for about 38-45 minutes if you are making is right away.
If you have just taken it out of the fridge, cover with aluminum foil.
There is no need to preheat the oven first. Just put it in the oven and turn it on to 350 degrees.
Bake for about 30 minutes, then remove the foil. Continue baking for another 15-30 minutes, or until the biscuits have browned and the sauce is bubbly. (45-60 minutes total if it's been in the fridge overnight).
Notes
*All of the spices I put in the eggs are totally optional. That's the way I like it, but you can add just salt and pepper if you want to keep it simple.