Preheat the oven to 375 degrees F.
Spray a muffin tin liberally with cooking spray. Put 4 or 5 tots into each cup, pressing down firmly to pack them in. Top each with a half slice of Cheddar, then crisscross with another half slice. Bake until crispy, 10 to 15 minutes.
Whisk the half-and-half and eggs in a bowl and sprinkle with salt and pepper. Divide the egg mixture among the "tot cups" and top each with another half slice of Cheddar and 1 slice of tomato. Bake until the eggs are firm, another 15 to 20 minutes. Let cool for 5 minutes, then carefully remove from the tin.
Top with avocado slices, sea salt flakes and freshly ground black pepper.