Preheat oven to 375°F. Grease a 9x13 inch baking dish.
Brown sausage in a skillet with onion until cooked and onions are soft. Drain fat.
Stir in chopped spinach until wilted, about 2 minutes. Remove from heat.
Spread tater tots in the dish as a base or top (I use them as a top here).
Whisk eggs, milk, salt, and pepper in a bowl. Stir in goat cheese and half the cheddar.
Spread sausage-spinach mixture over tater tots (or under if you prefer). Pour egg mixture evenly.
Sprinkle remaining cheddar on top. Bake 35–40 minutes until set and golden.
Let rest 5 minutes before slicing.