Place the oven rack in the center position and preheat the oven to 350º.
In a large bowl, use a whisk to beat eggs with milk, salt and pepper.
Heat olive oil in a 10-inch ovenproof skillet on medium. (Cast iron skillets work great)
Add yellow onion and asparagus to pan and sauté until soft, approx. 7 minutes.
Stir in spinach and sauté until wilted, approx. 2 minutes.
Spread the cooked vegetables evenly in a skillet and pour egg mixture over them.
Stir in ¼ cup of shredded cheddar cheese.
Crumble feta cheese on top.
Cook without stirring until eggs are just beginning to set around the edges, approx. 2 to 3 minutes.
Place the skillet in the oven and bake frittata until almost set in center, approx. 15 minutes.
Remove skillet from oven and top with remaining ¼ cup shredded cheddar cheese.
Set oven broiler to high and broil the frittata until top is golden brown, approx. 2 minutes. Keep a close eye on the pan to prevent burning.
Carefully remove the hot pan from the oven and let the frittata rest for 5 minutes before serving.
Notes
Additions: For instance you can add meat such as ham or cooked sausage. Top with fresh herbs to add add even more flavor.
Cheese: Feel free to switch up the type of cheese, try goat cheese or parmesan cheese, for example.
Vegetarian: This recipe is suitable for vegetarians as it contains no meat! Use up your leftover veggies for this simple recipe for an easy brunch anytime.
Gluten Free: If gluten free, this Frittata with Spinach and Feta is great option.
Sides: A nice green salad, fresh fruit, a yogurt parfait or english muffin,greek salad or use any extra spinach to make a salad, all will complement this easy frittata recipe.