Lightly butter both sides of bread slices and arrange them in a 9×13-inch baking dish.
In a nonstick skillet over medium-high heat, brown the beef sausage. Drain well and pat dry with paper towels. Sprinkle the sausage over the bread slices.
Next, layer the cheese over the sausage, followed by a layer of chopped green chilies.
In a medium bowl, lightly beat the eggs. Add salt, dry mustard, and half-and-half, and mix thoroughly. Pour this mixture over the casserole.
Cover the baking dish with foil and refrigerate it overnight.
When ready to bake, preheat the oven to 350°F and bake the casserole for 45 minutes, or until it becomes bubbly and lightly browned.
Allow the casserole to stand for 15 minutes to set before cutting it into squares.
Serve the squares with picante sauce on the side. Enjoy your delicious breakfast casserole!