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1
Preheat oven to 350 degrees (F). Position oven rack to the center of the oven.
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2
Heat 2 tablespoons olive oil in a 12-inch stainless steel skillet over medium-high heat until hot, but not smoking. Add the zucchini and cook, stirring often, until zucchini are nicely browned, about 5 minutes. Remove pan from heat and set aside.
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3
In a separate 12-inch nonstick ovenproof skillet, heat 1 tablespoon olive oil over medium heat. Add onion and saute until lightly browned, about 3 minutes. Add the zucchini, thyme, 1 teaspoon salt and ground black pepper. Continue to cook, stirring occasionally, for about 2 minutes more. Then remove from heat and set aside.
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4
Meanwhile, in a large bowl, whisk together the eggs, milk and remaining 1/2 teaspoon salt until well blended. Using a fork, stir in the cheese.
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5
Pour the egg mixture over the vegetables, gently lifting the vegetables to let the egg mixture coat the bottom of the pan. Reduce heat to medium-low heat and continue to cook, undisturbed, until sides are set , but top is slightly runny, about 10 minutes.
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6
While the frittata is slowly cooking, arrange tomato slices on top, slightly overlapping tomato slices in a circular pattern on top of the egg mixture to cover it entirely, starting from the outer edge of pan and ending in the center of the pan. Sprinkle with a few pinches of crumbled dried thyme just to give it some color.
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7
When frittata is finally set on the sides, transfer frittata to the preheated oven. Bake frittata until center is firm and not jiggly, about 25 minutes.
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8
Remove frittata from the oven. Change oven temperature to broil. Reposition oven rack to the top rack and broil the the top of the frittata for about 2 to 3 minutes to slightly brown the tomato slices. Let frittata cool for about 5 to 10 minutes before serving.
