Preheat oven to 375°F (190°C). Grease a 9x13-inch sheet pan (or line with parchment paper).
Sauté vegetables (optional): In a skillet over medium heat, cook onion, bell pepper, and jalapeños for 3-4 min.
Whisk eggs in a large bowl with milk, salt, pepper, and garlic powder until frothy.
Add mix-ins: Stir in sautéed veggies, tomatoes, and 1 cup of cheese.
Pour into sheet pan and spread evenly. Top with remaining ½ cup cheese.
Bake for 20-25 min or until center is set and edges golden.
Cool slightly, then slice into squares.
