2 teaspoons olive oil
1 cup shredded Gruyere or Swiss cheese
1 small onion, thinly sliced
4 large eggs, lightly beaten
1/2 teaspoon salt
1 pound asparagus, tough ends snapped off, spears cut diagonally into 1-inch lengths
Heat olive oil into a 10-inch oven-proof frying pan over medium high heat. Add onions and salt and cook, stirring occasionally, until onions are softened, about 3 minutes. Add asparagus, reduce heat to medium-low, and cook, covered, until the asparagus are barely tender, 6 to 8 minutes. Pour in eggs and cook until almost set, but still runny on top, about 2 minutes. While cooking, pre-heat oven broiler.
Sprinkle cheese over eggs and put in oven to broil until cheese is melted and browned, about 3 to 4 minutes. Remove from oven with oven mitts and slide frittata onto a serving plate. Cut into wedges.