Preheat the oven to 375 degrees.(convection setting if possible) Grease the bottom and the sides of a 9X13 inch baking dish with butter.
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 the egg and cheese mixture this into the buttered 9X13 inch baking dish.
Bake for 50 minutes covered with tin foil, then remove foil and add the remaining mozzarella cheese. Bake for another 15 minutes uncovered.