Preheat the oven to 350 degrees F (180 C). In a medium bowl, whisk together eggs and cream; set aside.
In a cast-iron or other oven-safe skillet, melt butter over medium-high heat. Add onion and cook for 1-2 minutes, until just beginning to soften. Add spinach and tomatoes, season with kosher salt and black pepper, and cook about 2 minutes more. Remove from heat.
Using a spatula or large spoon, spread veggie mixture evenly across the skillet's bottom. Sprinkle most of the cheese over the veggies, then pour egg mixture on top. Gently swirl the pan to ensure eggs are evenly distributed, then sprinkle remaining cheese on top.
Bake for 18-20 minutes, until eggs are set and the frittata begins pulling away from the edges of the pan. Allow to cool briefly, then slice and serve.
Last step!Please leave a review and rating letting us know how you liked the recipe. This helps my small business thrive and continue providing free recipes and high-quality content for you.
Notes
Substitutions: You can substitute half and half or milk for the cream, and swap out the veggies or types of cheese as desired. Frittatas are very flexible.
Yield: As written, this recipe is perfect for one 10-inch frittata, made in a standard 10-inch skillet. For us, this translates into 4 medium or 6 small servings. If you want to make a larger frittata, use a 12-inch skillet, 9 eggs, and 1/4 cup plus 2 tablespoons cream.
No oven-safe skillet? You can still make a frittata, but you'll have to transfer the sautéed veggies to a pie plate or similar dish before adding eggs and baking.
Storage: Frittatas keep very well. Store any leftovers tightly covered or wrapped in the fridge, and reheat briefly in the oven or microwave.