The combination of savory roasted veggies and nutty gruyere cheese with the creamy, cheesy egg base is to die for.. and SO simple! It is delicious for brunch, lunch, or dinner (or in my case, breakfast, dessert, snack, etc.. it is THAT good).
Toss peppers, zucchini, and onion with olive oil, 1/2 - 3/4 tsp salt, and 1/2 tsp pepper. Spread on baking sheet, and bake at 425 degrees for 15 minutes. Add the garlic, toss, and bake an additional 15 minutes. When you pull the veggies out, turn the oven down to 350 degrees to bake the assembled frittata.
While the veggies are roasting, mix the eggs, half and half, Parmesan, 1/2 - 3/4 tsp salt, and 1/2 tsp pepper in a bowl. Set aside.
In a 10" oven proof skillet, saute scallions in butter over med-low for about a minutes. Add veggies to the pan and toss. Pour egg mixture on top, and cook over med-low for 2-3 minutes without stirring. Transfer the skillet into the heated oven (be sure you turned it down to 350 degrees), and bake 20-30 minutes until puffed up and set in the center.
Sprinkle the cheese over the puffed frittata and return the pan to the oven for a few minutes just to melt the cheese. Slice and serve hot. As a note, this reheats great, and is even good cold!