Preheat the oven to 400 °F. Roast all the veggies and garlic on a baking sheet with some salt, pepper and olive oil for 20-25 minutes. Chop them into small pieces.
Meanwhile, in a large bowl, whisk together the eggs, half-and-half, Parmesan, 1 teaspoon salt, and ½ teaspoon pepper
In an ovenproof skillet, melt the butter and stir in the veggies over medium heat. They're all cooked so you just need to mix them in. Pour the egg mixture over the vegetables and cook until the eggs are mostly set but the center is still a little runny, 10 to 15 minutes.
Meanwhile, turn on your oven broiler to high.
Sprinkle the Gruyere cheese over the top and broil until the top is golden brown, about 3 minutes.
Let it sit in the skillet for about 10 minutes before cutting it into wedges.
Notes
You can substitute Swiss cheese if you can't find Gruyere cheese.
Recipe by Let's Cook Some Food at https://www.letscooksomefood.com/roasted-asparagus-and-pepper-frittata/