Preheat your oven to 375°F (190°C).
- Heat the olive oil in an oven-safe skillet over medium heat. Add the diced onion and sauté for 2-3 minutes until softened. Add the chopped asparagus and bell pepper and cook for 5-7 minutes until tender. Stir in the spinach and cook for an additional minute until wilted.
- In a bowl, whisk together the eggs, milk (or cream), salt, pepper, and garlic powder. Stir in the grated cheese.
- Pour the egg mixture over the vegetables in the skillet, ensuring the vegetables are evenly distributed. Let it cook on the stovetop for 2-3 minutes until the edges begin to set.
- Transfer the skillet to the preheated oven and bake for 12-15 minutes, or until the frittata is fully set in the center and golden on top.
- Remove from the oven and let it cool for a few minutes before slicing into wedges. Garnish with fresh herbs if desired. Serve warm or at room temperature.