Preheat the oven to 425 degrees.
Place the zucchini, peppers, and onion on a sheet pan. Drizzle with the olive oil, sprinkle with 1 1/2 teaspoons salt and 1/2 teaspoon pepper, and toss well. Bake for 15 minutes. Add the garlic, toss again, and bake for another 15 minutes. Remove from the oven and turn the oven to 350 degrees.
Meanwhile, in a large bowl, whisk together the eggs, half-and-half, Parmesan, 1 teaspoon salt, and 1/2 teaspoon pepper.
In a 10-inch ovenproof saute pan, melt the butter and saute the scallions over medium-low heat for 1 minute. Add the roasted vegetables to the pan and toss with the scallions. Pour the egg mixture over the vegetables and cook for 2 minutes over medium-low heat without stirring. Transfer the pan to the oven and bake the frittata for 20 to 30 minutes, until puffed and set in the middle. Sprinkle with the Gruyere and bake for another 3 minutes, until the cheese is just melted. Cut into 6 or 8 wedges and serve hot.
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