Grease an 8-inch square baking dish.
Place four tortillas over the bottom of the dish. (They will overlap just a bit and likely leave a little open space right in the middle of the dish, which is ok.) Allow the tortillas to extend up the sides of the dish by about an inch. Cut the fifth tortilla in half, and then cut each half into strips. Set the strips aside.
Sprinkle ½ cup of the cheese over the four tortillas, and then top with the cooked sausage.
In a medium bowl, combine the eggs, milk, oregano, salt, pepper, and optional chopped green chilies; whisk to thoroughly combine. Pour over the mixture in the baking dish, and then arrange the cut tortilla strips on top. Sprinkle with the remaining cheese.
If using the jalapeño pepper, thinly slice it and distribute the rounds over the cheese. (For less heat, slice the pepper in half first and remove the seeds and veins. Then slice the two halves and evenly distribute the pieces.)
Tightly cover the baking dish with foil and refrigerate 8 hours or overnight.
When ready to eat, allow the casserole to sit on the counter while the oven preheats to 350℉. Bake the covered casserole for 30 minutes, or until the center is just short of set.
Uncover, and bake an additional 5 minutes or until the center of the casserole is just cooked through.
Allow the casserole to sit for 5 minutes before serving. Serve as is or with optional toppings of choice.