Preheat oven: Start by preheating your oven to 350°F (175°C).
Sauté vegetables: In a skillet over medium heat, add olive oil. Once hot, toss in the mixed vegetables and sauté until they are tender, about 5–7 minutes.
Whisk egg mixture: In a large bowl, whisk together the eggs, milk, salt, and pepper until well combined.
Add vegetables: Stir in the sautéed vegetables along with half of the shredded cheese into the egg mixture.
Transfer to a baking dish: Pour the entire mixture into a greased baking dish to ensure it doesn’t stick.
Top with cheese: Sprinkle the remaining cheese on top for an extra cheesy finish.
Bake: Place the dish in the preheated oven and bake for 25-30 minutes, or until the eggs are set and the cheese is golden brown.
Cool and serve: Allow the casserole to cool slightly before cutting into squares. Serve warm and enjoy!