800 g Swiss brown button mushrooms

3 onions, sliced into rings

2 tbsp olive oil

1 tsp sea salt

½ tsp cracked black pepper (plus extra to serve)

2 tbsp melted butter

2 tsp soy sauce

2 tsp balsamic vinegar

1 tbsp garlic, finely minced

2 tsp fresh thyme (or 1 tsp dried)

1 tsp chopped fresh parsley