How to Make It
Preheat oven to 450°F. Once oven is fully preheated, place a 10-inch cast-iron skillet in oven for 10 minutes.
Meanwhile, place eggs, milk, flour, pepper, half the cheese, and ½ teaspoon salt in a blender. (Do not blend yet.)
Remove skillet from oven. Add butter to hot skillet, swirling until melted. Add spinach and remaining ¼ teaspoon salt and toss to coat. Return skillet to oven and bake until spinach is wilted, about 3 minutes.
Meanwhile, start the blender. Blend egg mixture until very frothy, 1½ to 2 minutes.
Remove skillet from oven. Using tongs, transfer spinach to a plate. Pour off any liquid in skillet. Pour batter into hot skillet and top with spinach and remaining cheese. Return to oven and bake until puffed and golden, 15 to 18 minutes. Serve immediately.