Heat an oven-safe large (10 inches) non-stick skillet on the stove over medium heat.
Prepare your vegetables. Wipe off and slice the mushrooms. Cut the dark green leaves of the leek off and cut off the root end (you will only use the white and light green parts). Cut the leek lengthwise in half and slice one half into strips approximately 1/4 inch wide. Put all of the cut leek into a bowl with water and swish the leek around to remove the sand and dirt. Let it sit until you are ready to use the leek.
Trim the root end of the asparagus and cut into 2 inch pieces.
Add the olive oil to the skillet. When hot, add the mushrooms, leeks, and asparagus. Saute the vegetables until they are just tender, approximately 5 minutes.
While the vegetables are sauteeing, in a medium bowl whisk together the eggs, Parmesan cheese, salt, and black pepper.
When the vegetables are ready, add the egg mixture to the skillet. Stir very lightly. Let the egg mixture sit and cook on the stove until the edges are just set, approximately 5 minutes.
Turn the stove off and place the skillet in the oven. Let the frittata cook until it is cooked through and dry to the touch on top, approximately 10-12 minutes.
Use a rubber spatula to loosen the frittata and either slide out or flip onto a plate. Serve immediately.
Notes
The smaller the skillet, the thicker your frittata will be and the longer it will need to cook. The larger the skillet, the thinner the frittata and the faster it cooks. I use a 10 inch skillet for this recipe.A non-stick skillet is best, but you can also use a cast iron skillet. The egg tends to stick more in cast iron, making it difficult to flip out if you choose to do so.To ensure a complete vegetarian frittata, use vegetarian cheese.