In a 10-inch nonstick skillet with an ovenproof handle, cook white part of scallions and garlic in 2 tablespoons of oil over moderate heat, stirring, until softened, about 2 minutes. Add potato and cook over moderately low heat, stirring until tender, about 10 minutes. Add corn and salt and pepper to taste, and then cook, stirring, about 1 minute for thawed corn or 3 minutes for fresh corn.

Preheat broiler.

Whisk together eggs, mozzarella and salt and pepper to taste. Stir in potato mixture and scallion greens. Wash skillet.

Heat remaining tablespoon of oil in the skillet over moderate heat until hot, but not smoking. Pour in potato-egg mixture, and cook without stirring, shaking the skillet once or twice to loosen frittata, until underside is golden but top is still wet. Broil frittata about 3 inches from heat until top is just set and golden, about 2 minutes. Slide onto a plate and cool to warm or room temperature.