How to Make It
Set an oven rack about 6 inches from heat source; preheat broiler to high.
Warm oil in a large, ovenproof nonstick skillet over medium-high heat. Add kale and cook, stirring, until wilted, about 3 minutes. Add scallions; sauté for 1 minute. Stir in ham; sauté 1 minute longer.
Whisk eggs and cheese; season with salt and pepper. Arrange kale mixture evenly in skillet and pour egg mixture over. Reduce heat to medium-low and cook without stirring until frittata is almost set in center, 10 to 12 minutes.
Transfer skillet to broiler and cook, watching carefully to avoid burning, until top is set and golden brown, 1 to 2 minutes. Run a spatula around edges of skillet and cut into wedges (to safeguard your pan, you can slide frittata onto serving plate before slicing). Serve hot, or allow to cool and serve at room temperature.