Preheat the oven to 350°F. Spray a 9 X 13-inch casserole dish with cooking spray and set aside.

Heat a large skillet over medium-high heat and add the sausage. Cook until the sausage is mostly browned, about 5 minutes. Add the onion, red bell pepper, and green bell pepper then season with the kosher salt and freshly ground black pepper. Turn the heat to medium and cook, stirring, until the vegetables soften, about 5 minutes. Remove the meat mixture from the pan and transfer to a large bowl or baking sheet to cool.

In a 1 quart measuring cup, whisk the eggs and milk until smooth. In a small bowl, use a fork to mix 1 tablespoon of the egg and milk mixture into the dry mustard and mix until smooth. Add the mustard mixture into the quart measuring cup and mix well so that all of the mustard is incorporated. Set aside.

Layer the casserole. Tear the bread slices into 1-inch pieces of bread. Cover the bottom of the baking dish, fitting the pieces tightly together like puzzle pieces. Add ¾ of the sausage and pepper mix over the bread. Starting at one end of the casserole, slowly pour the egg mixture over the bread and sausage mixture, evenly distributing and filling in around the casserole edges

Toss the cheddar cheese and frozen hash browns together in a bowl and add to the top of the casserole. Sprinkle with the rest of the sausage mixture.

Bake for 60-70 minutes or until the eggs are set in the middle and bubbling at the edges. If the casserole begins to brown too much, loosely tent foil over the top and continue to cook until the egg is set. Remove the casserole from the oven and let sit for about 10 minutes before cutting and serving.