In a 20cm shallow ovenproof frying pan over medium high heat; heat the olive oil and add the pancetta. When the pancetta has taken some colour add the asparagus and onion. Saute until the asparagus is tender and onion translucent.
Reduce heat to medium low. Add the whisked eggs then season with salt and pepper to taste. Stir gently to incorporate with the asparagus mixture.
Cook over medium low heat until the bottom of the frittata is set. Meanwhile preheat the grill.
When the bottom of the frittata is set sprinkle the crumbled feta over the top. Place the frittata about 8 to 10cm under the grill.
Cook until the top is lightly browned and the middle is set; about 4 to 6 minutes. Watch carefully so as not to burn.
You can easily make this vegetarian by omitting the pancetta. Similarly, if you haven't any pancetta, use diced streaky bacon instead.