Slice the mushrooms and finely chop the garlic.
Add the oil and garlic to a large frying pan.
When the oil is hot and garlic is sizzling, add the mushrooms and stir for a minute. Add a couple of pinches of chilli flakes, if used.
Cover the pan with a lid, turn down the heat and cook for 5 minutes.
Add the vinegar. Put the cover on again and cook until the mushrooms have softened and released their water. This should take about 10 minutes over low heat.
Season with salt and pepper then add the watercress, and turn off the heat.
Cover with a lid, then rest for 5 minutes until the watercress has softened, then combine well.
Serve with some extra chilli flakes, if liked.
For a vegetarian version you can add some grated Parmesan or goat’s cheese, feta etc.