Brown both packages of sausage together in a skillet and set aside.
Crack all 12 eggs in a bowl, add milk or cream, salt and pepper to taste and ground mustard. Whisk together vigorously.
Butter the bottom and sides of a 13X9 pan. Tear bread slices and place in the bottom. The amount of bread is up to you. Other recipes give a specific slice count, I don't use the same number from one time to the next. It will depend on the kind of bread you pick. It is alright to see the bottom of the pan, as in not completely covered.
I like to sprinkle some shredded cheese over the bread then layer half of the sausage, velveeta slices, and the remaining sausage.
Add the egg mixture over the entire pan and use a wooden spoon to push any bread back into the egg mixture. Sprinkle shredded cheese on top and cover pan to refrigerate over night.
Preheat you oven to 375 degrees and place covered casserole in oven for 45 minutes. Reduce oven temp to 350 degrees and cook uncovered for 15-20 minutes.