01 - Set oven to 400°F (200°C) and allow it to fully preheat.
02 - Dice zucchini and red bell pepper into small, even cubes.
03 - Heat olive oil in an ovenproof skillet over medium heat. Add zucchini and bell peppers, sautéing until just tender, about 4 minutes.
04 - In a bowl, whisk eggs with milk, salt, and black pepper until smooth. Stir in shredded cheddar cheese.
05 - Pour egg mixture evenly over sautéed vegetables in the skillet. Stir gently to distribute ingredients.
06 - Transfer skillet to preheated oven. Bake for 10-12 minutes until eggs are just set and surface is lightly golden.
07 - Allow frittata to cool briefly before slicing and serving warm.