Preheat oven to 350°F. Grease a 9×13-inch (glass) baking dish with a little of the butter or nonstick spray.
In a large skillet over medium heat, melt the 2 tablespoons butter.
Add the 1/2 large diced onion, 1 diced green bell pepper, and 1 diced red bell pepper to the skillet. Sauté 5–7 minutes, stirring occasionally, until the vegetables are softened. Remove from heat and let cool slightly.
Crack the 12 large eggs into a large bowl. Season with salt and pepper to taste and beat until combined.
Stir the cooked vegetables and 1 cup of the grated cheddar cheese into the beaten eggs.
Pour the egg–vegetable mixture into the greased 9×13 dish and spread evenly.
Arrange the 3 cups frozen tater tots in a single layer on top of the egg mixture, pressing them slightly so they make contact with the eggs.
Sprinkle the remaining 1/2 cup grated cheddar cheese evenly over the tater tots.
Bake uncovered at 350°F for 30–35 minutes, or until the eggs are set (the center should not jiggle) and the cheese is melted and beginning to brown.
Remove from the oven, let rest 5 minutes, then top with chopped cilantro. Cut into pieces and serve warm with salsa and sour cream (and any other extra toppings you like).