2 medium zucchini or other summer squash, coarsely grated
2 tablespoons olive oil, divided
1 small yellow onion, thinly sliced
2 garlic cloves, thinly sliced
Kosher salt, freshly ground pepper
1 tablespoon unsalted butter
12 large eggs, beaten to blend
1/3 cup crème fraîche or sour cream
1/4 cup ricotta
2 tablespoons finely grated Parmesan
1 1/2 cups (packed) mixed fresh tender herbs, such as basil, chives, parsley, and tarragon
2 teaspoons finely grated lemon zest
1 tablespoon fresh lemon juice
Crushed red pepper flakes (optional; for serving)
Preheat oven to 350°. Using your hands and working in batches, squeeze excess liquid out of zucchini (this is key to avoid a soggy frittata-do your best).
Heat 1 Tbsp. oil in an 8″ or 10″ cast-iron skillet over medium-high. Add onion and garlic, season with salt and pepper, and cook, stirring occasionally, until onion is softened and beginning to brown, 5-8 minutes. Add zucchini and season with salt and pepper. Cook, stirring occasionally, until zucchini is cooked through and beginning to brown (mixture will look quite dry at this point), 5-8 minutes. Reduce heat to medium-low and stir in butter until melted.
Whisk eggs and crème fraîche in a large bowl and season with salt and pepper; pour over vegetables. Cook, stirring egg mixture and swirling pan frequently (be sure to scrape down the sides as well), until edges are just set (the texture of a very soft scramble), about 5 minutes. Dollop with ricotta and sprinkle with Parmesan. Transfer skillet to oven and bake eggs until puffed slightly and the center is set, 12-15 minutes. Let frittata cool in pan at least 10 minutes.
Meanwhile, combine herbs, lemon zest, lemon juice, and remaining 1 Tbsp. oil in a small bowl. Season herb salad with salt and pepper and arrange over fritatta. Top with red pepper flakes, if using, and slice into wedges before serving.