Frittata - Scallion and Mushroom

2 tbsp olive oil
12 scallions/green onions (including green parts) cut into 1.5 inch pieces
1/2 lb shiitake or other mushrooms, sliced, stems discarded or finely diced
salt and pepper
8-12 eggs (depending what else you put in)
1/2 cup sharp cheddar cheese
1/2 cup fresh grated Parmesan cheese
Other filling options: A bunch of spinach, diced and de-stemmed, bell peppers, cubes of ham, bacon, sundried tomatoes, chives, squash, zucchini, kale, sausage, diced potato leftovers
Some recommend 1/4 cup heavy cream for every 6 eggs used, to increase fluffiness. We often omit it.

Preheat oven to 350 degrees
Heat olive oil in a cast iron skillet (about 8 inches in diameter, 1.5 inches deep minimum), sautee the mushrooms and scallions and other vegetables until all the moisture is gone (~15 minutes)
In large bowl, whisk the eggs (and cream, and cheddar cheese or other cheese) together with 1/4 tsp salt, pepper.
Once the vegetables are done, pour them into the bowl with the egg mixture, stir in, then replace into the skillet (to ensure that there isn't a layer of veggies and a layer of eggs...)
Cook on the skillet for about 3-4 minutes, add the grated parmesan on top of the frittata, then move to the oven for 15 minutes
To keep from over cooking, after 10-15 minutes, turn the oven up to 450 if the top isn't browned to quickly brown it, then remove