Cut the tough ends of the asparagus spears and discard. Chop the remaining spears into ¼” pieces, reserving six of the tips. You should have about ¾ cup chopped asparagus.
In a small skillet, heat two tablespoons of the olive oil and add the asparagus and the green onion. Season with a little salt and cook for just a few minutes to soften the asparagus. Remove from the heat and allow them to cool.
Whisk the eggs, milk or cream, and the salt and pepper in a large measuring cup until well blended (this will make the pouring easier).
Brush all six cups of a regular muffin tin generously with the remaining olive oil. If you have a non-stick one muffin tin, all the better.
Divide the asparagus and onion mixture evenly among the 6 muffin cups. Add the Asiago cheese on top of the vegetables. Next, pour the egg and milk mixture over the filling in each cup as evenly as possible, about two-thirds of the way from the top. Add the asparagus tips to top each muffin.
Bake until the egg mixture puffs up and is soft but set in the center, about 15 minutes or so. Do not overcook them (the color of the egg custard should not be brown.)
Sprinkle the top with some Parmesan cheese and allow them to sit for a few minutes. Using a rubber spatula, loosen the frittate from the muffin cups and slide them onto a serving platter. Sprinkle with some fresh parsley and serve immediately.
Recipe by Marilena's Kitchen at https://marilenaskitchen.com/easy-baked-frittate-with-asparagus-and-asiago-cheese/