1. Preheat oven to 375 degrees F.
2. Heat oil in a 12-inch nonstick skillet over moderately high heat until hot but not smoking, then saute zucchini with salt and pepper, stirring occasionally, until just tender, about 4 minutes.
3. Whisk chives, zucchini, and 1/2 cup cheese into eggs. Divide mixture among oiled muffin cups and bake in middle of oven until tops are puffed and set, about 14 minutes. Remove pan from oven and turn on broiler. Sprinkle frittatas with remaining 1/4 cup cheese and broil 3 to 4 inches from heat until cheese is melted and tops are golden, 1 to 2 minutes.

Yield: 6 servings