Step 1

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).

Step 2

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.

Step 3

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.

Step 4

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.