Thinly slice the mushrooms, cut the asparagus in half lengthwise. Crumble the feta.
Beat the eggs in a measuring cup. Finely chop the chives with scissors, whisk into the eggs along with the salt and pepper.
Heat the oil in a non-stick frying pan. Stir fry the mushrooms and asparagus for approx. 5 mins., season with salt. Reduce the heat. Pour in the egg mixture. Scatter the feta on top. Cover the frittata and leave to solidify for approx. 15 mins.