Preheat the oven to 375 degrees.
Rub the butter all over the bottom and up the sides of a 9X13 inch baking dish.
Fry the sausage and onion together in a large skillet. This will take about 10 minutes, make sure there are no traces of pink left in the meat.
In a large bowl, mix tater tots, sausage, and onion.
In a separate bowl, beat eggs slightly, add milk, half and half, seasoning salt, pepper, garlic, cheddar cheese, pepper jack cheese, and half of the mozzarella cheese.
Add this mixture to the tater tot and sausage mixture.
Pour all of this into the buttered 9X13 inch baking dish.
Bake for 45 minutes covered with tin foil, then remove foil and add the remaining mozzarella cheese.
Bake for another 15 minutes uncovered.