Put the potatoes in a pan of water then bring to the boil. Cook for 10- 15 minutes or so, until they are tender, then drain.
Meanwhile, thinly slice the onion and garlic.
Heat the oil in a large frying pan, then gently sauté the onion until soft, then add the garlic and thyme leaves and stir for a minute.
Add the mushrooms and season to taste. Add the lemon juice (and mustard or chilli if used), put a lid on and gently sauté for about 10 minutes.
Cut the drained potatoes into bites-sized pieces, if needed.
Add these to the mushroom mixture, together with the chopped parsley and toss well.
Season to taste, then serve.