Preheat broiler. Pour olive oil into a 10-inch ovenproof frying pan over medium-high heat. Add onion and salt and stir until onion is softened but not browned, about 3 minutes. Add asparagus, reduce heat to medium-low, and cook, covered, until asparagus is barely tender, 6 to 8 minutes. Pour in eggs and cook until almost set but still runny on top, about 2 minutes.
Sprinkle cheese over eggs and broil until cheese is melted and browned, 3 to 4 minutes. Slide frittata onto a serving platter and cut into wedges.
Note: Nutritional analysis is per serving.