Preheat the oven to 350° F.

Place 4 large eggs in a large bowl and whisk until blended.

Add 1/3 cup whole milk, cheddar cheese, 1/2 teaspoon kosher salt, and 1/4 teaspoon black pepper.

Whisk to combine.

Heat 2 tablespoons olive oil in a 10-inch cast iron or oven-safe nonstick skillet over medium heat until shimmering.

Add the onion and sauté until softened, about 3 minutes.

Add the baby spinach a handful at a time, and sauté until just wilted, about 2 minutes.

Spread the vegetables into an even layer, then pour the egg mixture over the top.

Tilt the pan to make sure the eggs settle evenly over all the vegetables.

Sprinkle with the Parmesan.

Cook undisturbed until the eggs at the edges of the pan begin to set, about 2 minutes.

Transfer the skillet to the oven and bake until the eggs are just set, 18 to 20 minutes.

To check, cut a small slit in the center of the frittata.

Let cool in the pan for 5 minutes, then slice into wedges and serve warm or at room temperature.