Preheat oven to 400°F (200°C).
Cut peppers lengthwise, remove seeds and membranes.
Place peppers in a baking dish, cut-side up.
Whisk eggs, milk, salt, and pepper in a bowl.
Add diced tomatoes, green onions, and half the cheese to egg mixture.
Pour mixture into pepper halves, filling to just below the rim.
Top with remaining cheese.
Bake for 20-25 minutes until eggs are set.