Cooking time for a frittata is usually around 8 to 10 minutes, however several factors can play into delaying or speeding the process like the width of your burner, the thickness of your pan or even how tight the lid is to the pan.
In a large bowl beat together eggs, milk and Parmesan cheese, season with salt and pepper.
Melt the butter in a large sauté pan over medium high heat, move the pan around to make sure its bottom gets evenly coated.
Pour the egg mix into the pan, again swirl gently the pan around to obtain an even layer of ingredients.
With your hands, gently place the baby greens on the eggs, pressing on them just enough to sink slightly into the egg batter. Cover the entire eggs surface with greens then cover with a lid and reduce flame to a medium-low.
Cook 8 to 10 minutes; checking every now and then is the best way to ensure great results.
Frittata should look slightly sweaty in its center, but not stick to your finger if you touch it, the edges should be crispy, almost caramelized, all the vegetables will be wilted into the eggs and most importantly, well cooked frittatas always slide easily out of the pan into a serving vessel.