Step 1

Preheat the oven. Arrange a rack in the top third of the oven and heat the oven to 350°F.

Step 2

Prepare the shallot and cheeses. Dice 1 medium shallot. Grate 3 ounces sharp cheddar cheese on the large holes of a box grater (about 3/4 cup). Finely grate 1/2 ounce Parmesan cheese on the small holes of a box grater (about 1/4 packed cup).

Step 3

Whisk the eggs and dairy together. Place 8 large eggs in a large bowl and whisk until blended. Add 1/3 cup whole or 2% milk, cheddar cheese, 1/2 teaspoon kosher salt, and 1/4 teaspoon black pepper. Whisk to combine.

Step 4

Sauté the shallot and spinach. Heat 2 tablespoons olive oil in a 10-inch cast iron or oven-safe nonstick skillet over medium heat until shimmering. Add the shallot and sauté until softened, about 3 minutes. Add 5 ounces baby spinach a handful at a time, and sauté until just wilted, about 2 minutes.

Step 5

Pour in the egg mixture. 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.

Step 6

Bake the frittata. 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. If raw eggs run into the cut, bake for another minute or two. If not, then the eggs are set.

Step 7

Broil the frittata. Heat the broiler to HIGH. Broil until the top of the frittata is lightly golden-brown in spots, 2 to 3 minutes. Let cool in the pan for 5 minutes, then slice into wedges and serve warm or at room temperature.