Arrange onions and zucchini in two baking dishes, giving them space so they’ll brown nicely. Drizzle them with oil and roast at 400 degrees F for 45 min – 1 hour. Halfway through, pull them out of the oven and flip every vegetable over, then continue roasting.
Turn the oven down to 375 degrees F.
Beat the eggs until they’re nice and frothy.
Place butter in a glass baking pan and place in oven briefly until butter is melted.Spread the butter around so it covers the bottom of the pan. Add the eggs and then the vegetables.
Bake for 20-25 minutes, until the eggs are no longer runny.
Carefully pull the pan out of the oven, sprinkle with Feta and return the pan to the oven for another 3-5 minutes to toast the cheese.