Roast them
Oven at 200C/180C fan/gas 6.
Arrange the mushrooms, say 500g in a roasting tray in a single layer, then cook for 15 mins.
scale the quantities according to what you have in mushrooms.
Meanwhile, mix the garlic (3 cloves or adjust to taster), rosemary (1 tblsp - I love lemon thyme in this too), olive oil (2 tblsp) and a tblsp of balsamic vinegar with some seasoning. R
Remove mushrooms and pour away any liquid, then toss in the oil and herb mix.
Return to the oven for 10-15 mins more until tender. Scatter with parsley if you have any before serving.