Slice the mushrooms and onion.
Heat the oil in a large frying pan. Add the onion and sauté for a few minutes.
Add the garlic and the mushrooms and stir for a minute. Add the chilli flakes, cover the pan with a lid, turn down the heat and cook for 5 minutes.
Add the lemon juice, let it sizzle then pour over some water (about half a cup). Cook for another 5 minutes or so, until the mushrooms are soft.
Crumble in the cheese and stir gently until it has melted and the sauce has thickened.
Fold in the rocket leaves, season with pepper and some extra chilli flakes, if you like.
Serve warm.