• Preheat broiler. Pour olive oil into a 10-inch ovenproof frying pan over medium-high heat.
• Add onion and salt and stir until onion are 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.
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