Preheat oven to 350 degrees F. Grease a 12-cup muffin pan. If you prefer to use liners, spray them with cooking spray so they don’t stick. Optionally, grease a 9-inch square baking dish.
In a large bowl, beat the eggs, and then add the quinoa, chopped vegetables of choice, crumbled feta, salt and pepper. Stir to thoroughly combine.
Divide the egg mixture among the prepared muffin cups or transfer to the baking dish. When using the muffin pan, I find it less messy to use a large ice cream scoop to transfer the egg mixture.
Bake the individual frittatas for approximately 20 minutes and the casserole for 25 minutes, or until the center is just barely firm to the touch. After 15 minutes for the muffin pan or 20 minutes for the baking dish, remove from the oven and sprinkle the shredded cheese on top, returning to the oven to melt the cheese and finish cooking. If the center is still jiggly, cook and few minutes more and check again.
Serve hot or at room temperature. If making ahead, the frittatas may sit at room temperature for up to 2 hours. Beyond 2 hours, cover, refrigerate, and gently reheat.