Preheat the oven to 425ºF. In a large bowl, whisk together the eggs and 1/4 cup of the pesto, 3/4 teaspoon salt, and a few grinds of pepper. Set aside.

Add 2 tablespoons of the oil to a 10-inch ovenproof skillet and heat over medium-high heat until shimmering. Add the onion and sauté for 3 minutes. Add the zucchini and cook for 5 minutes, stirring often, then add the tomatoes, and cook 2 minutes longer. Stir the egg mixture, then pour over the veggies in the hot skillet. Let cook, undisturbed, for 3 minutes.

Transfer the skillet to the oven and bake until the frittata is just set and no longer runny on top, 12 to 15 minutes.

Meanwhile, toss the microgreens with the 2 teaspoons oil and the lemon juice. Season to taste with salt and pepper.

Cut the frittata into wedges, transfer to plates, top each wedge with a spoonful of the remaining pesto, and serve warm with the microgreens.