Skin then dice the chorizo. Thinly slice the onion. Put them in a frying pan and gently heat the pan over low heat. Allow the sausage release its fat and soften the onion in it.
Meanwhile, chop the red pepper and thinly slice the mushrooms. Add them to the pan. Put a lid on and gently sauté for 5 minutes, then add the vinegar and cook for another 5 minutes.
Lightly whisk the eggs with a couple of pinches of salt.
When the vegetables are tender, pour over them the egg mixture, letting it spread out evenly.
Put a lid on and cook for about 10 minutes over low heat, until the surface has almost set.
Meanwhile, turn on the grill in your oven.
Put the pan under the grill for a couple of minutes to brown the top. (If liked, scatter some cheese on the top before putting it in the oven).
Slide the frittata onto a plate, then cut it into wedges and serve immediately with the wild rocket.
Alternatively, let it cool completely and cut it into small squares and serve as tapas.