Preheat the oven to 350 degrees F. (Unless baking it the next day.)
Line the tater tots in a buttered or greased 9-by-13-inch baking dish.
Add the olive oil to a large skillet over medium heat. Add the sausage and cook, breaking the sausage up with a wooden spoon, until browned and cooked through about 8 minutes. Add the onion and saute until soft, about 3 minutes. Drain the grease, then set aside to cool slightly.
Sprinkle the sausage and onions over the tater tots. Sprinkle the diced bell peppers evenly over that. Then sprinkle half of each type of cheese.
In a large bowl, mix the eggs, milk, half-and-half, salt, pepper, and seasoned salt. Pour over the tater tots and sausage mixture, then top with the rest of the cheese. Cover with foil. Put in the fridge overnight or cook right away.
Bake, covered, for 30 minutes. Remove the foil and continue to bake until the cheese is brown and bubbly and the casserole is cooked through another 25 to 35 minutes. Cut into squares and serve.