Preheat and Prep: Start by preheating your oven to 375°F (190°C). This ensures a gentle, all-around heat that will perfectly set your frittata.
Sauté the Veggies: In a 10-inch oven-safe skillet, heat the olive oil over medium heat. Add the chopped onion and broccoli. Sauté until the onions are translucent and the broccoli is bright green but still crisp, about 5 minutes. This step is crucial for layering flavors.
Whisk: In a large bowl, whisk the eggs, milk, salt, and pepper until well combined. The secret to a fluffy frittata is in the whisking - the more air you incorporate, the lighter it will be.
Combine and Cook: Stir the sautéed vegetables into the egg mixture, then fold in the shredded cheddar. Pour the mixture back into the skillet. Cook over medium heat until the edges begin to set, about 2-3 minutes.
Bake: Transfer the skillet to the oven and bake for 12-15 minutes, or until the frittata is set and the top is golden. The oven does the heavy lifting here, turning your liquid egg into a puffy delight.
Garnish and Serve: Let it cool for a few minutes, then garnish with fresh herbs if using. Slice and serve directly from the skillet for a rustic presentation that's sure to impress.