Mostly Greens Frittata

Melt butter in an 8-inch cast-iron skillet. Add garlic; cook until soft, 1 minute. Add spinach; heat through. Season whisked eggs with salt and pepper. Add to skillet and cook, pushing eggs into center to create curds, until almost set, 3 minutes. Sprinkle with Gruyere. Bake at 400 degrees until set and golden, 12 minutes.