Preheat oven to 375°F; grease a 9x13 inch baking dish.
Brown sausage with onion in a skillet until sausage is cooked and onion soft. Drain fat.
Stir in spinach until wilted, about 2 minutes. Remove from heat.
Arrange tots in dish as a base, or reserve for topping.
Whisk eggs, milk, salt, and pepper; stir in goat cheese and half the cheddar.
Spread sausage-spinach mixture over tots, or beneath them if using as topping. Pour egg mixture evenly.
Sprinkle with remaining cheddar. Bake 35–40 minutes until set and golden.
Rest 5 minutes before slicing.