Preheat your oven to 375°F (190°C). This ensures the frittata will cook evenly and set perfectly once it goes into the oven.
In a medium oven-safe skillet, heat the olive oil over medium heat. Add the diced onion and bell peppers and sauté for about 5 minutes until they begin to soften. Then, toss in the chopped spinach and halved cherry tomatoes. Cook for another 2-3 minutes until the spinach wilts and the tomatoes soften slightly. Season with a pinch of salt and pepper to enhance the flavors.
While the veggies are cooking, crack the eggs into a large bowl. Add the milk, a pinch of salt, and pepper, then whisk vigorously until the mixture is smooth and slightly frothy. This will help make your frittata nice and fluffy.
Pour the egg mixture over the sautéed veggies in the skillet, making sure it’s evenly distributed. Sprinkle the crumbled goat cheese across the top, letting it nestle into the eggs.
Allow the frittata to cook undisturbed over medium heat for about 4-5 minutes. You want the edges to start setting but the center should still be slightly runny.
Transfer the skillet to the preheated oven and bake for 12-15 minutes, or until the eggs are fully set and the top is lightly golden. To check doneness, gently shake the pan—the center should no longer jiggle.
Remove the skillet from the oven and let the frittata cool for a few minutes before slicing. Serve warm or at room temperature for a delightful meal anytime.