Seasonal Frittata Recipe

Slice onions

Peel and cut the yellow onions into thin slices (about 1/8-inch). Reserve 2 cups sliced onions (about 2 medium onions).

Caramelize onions

In a large (12-inch) ovenproof skillet or sauté pan, heat 2 tablespoons extra-virgin olive oil over medium-low heat. Add the sliced onions and 1/4 teaspoon kosher salt. Cook gently, stirring occasionally, until soft and deep golden brown, about 25–30 minutes. If onions start to brown too quickly, lower heat and add 1–2 tablespoons water to deglaze. When done, remove half the caramelized onions to a bowl for topping and leave the rest in the pan if using them distributed in the frittata.

Prepare seasonal vegetables

While onions caramelize, chop other vegetables: 1 medium zucchini into 1/2-inch rounds, 1 red bell pepper into 1/2-inch dice, 1 cup trimmed asparagus into 1-inch pieces, and halve 1 cup cherry tomatoes. If using spinach, roughly chop 3 cups. Pat vegetables dry.

Sauté vegetables

After removing reserved caramelized onions, add 1 tablespoon olive oil to the same skillet and increase heat to medium. Add heartier vegetables first (zucchini, bell pepper, asparagus) and sauté 4–6 minutes until just tender. Add garlic and cook 30 seconds. If using spinach, add it last and cook until wilted, 1–2 minutes. Season vegetables with remaining 1 1/4 teaspoons kosher salt and 1 teaspoon black pepper. Spread vegetables in an even layer in the pan and let cool slightly.

Preheat the oven to 375°F (190°C). In a large mixing bowl, whisk together 18 large eggs, 1 1/2 cups whole milk or half-and-half, 1 teaspoon salt, 1/2 teaspoon black pepper, and 1 teaspoon Dijon mustard (if using) until fully combined and slightly frothy. Stir in 1/4 cup chopped fresh herbs.

Stir in cheese: Fold 1 1/2 cups of the grated cheddar (reserve 1/2 cup for topping) and the optional 1 cup crumbled goat cheese into the egg mixture, leaving some for sprinkling on top.

Temper eggs (optional): If the skillet is very hot, remove briefly from heat and let cool 1 minute so eggs don't scramble on contact. Slowly pour the egg-cheese mixture over the sautéed vegetables in the skillet, tilting the pan so eggs distribute evenly.

Top with flavors: Scatter the reserved caramelized onions across the top of the custard, then sprinkle the reserved 1/2 cup grated cheddar and the lemon zest (if using) evenly.

Transfer to oven: Bake uncovered in the preheated 375°F (190°C) oven until the frittata is set around the edges and slightly wobbly in the center, about 20–25 minutes. For a golden top, switch to broil for 1–2 minutes, watching closely to prevent burning.

Rest: Remove from oven and let rest 8–10 minutes so the custard finishes setting. The frittata will firm up as it cools.

Finish: Sprinkle remaining fresh herbs and optional crumbled goat cheese over the top before slicing. Cut into 12 rectangular or wedge portions for catering service.

To serve warm: Reheat individual portions in a 350°F (175°C) oven for 8–10 minutes or in a microwave for 60–90 seconds.

Make-ahead: Assemble and keep covered in the refrigerator up to 24 hours before baking. Alternatively, bake fully, cool, and refrigerate up to 3 days; reheat before service. For freezing, wrap tightly and freeze up to 1 month; thaw overnight in refrigerator before reheating.