I always used to cook my whole mushrooms in a frypan (skillet) but I think that I have gotten lazier as I get older because now I usually just stick them in the oven. I will put both methods below so you can choose:
Frypan Method (Skillet)
Melt margarine and olive oil over a medium heat.
Remove stems from mushrooms.
Add mushrooms to pan top side down. You will probably need to cook in a couple of batches.
When starting to cook and soften on top side, sprinkle with sea salt and turn over and repeat on other side. Sprinkle again with sea salt.
As they cook, juice form the mushrooms will leak into the pan. You just need to keep an eye on them so the pan doesn’t get too dry (if it does, add extra margarine) or that there isn’t too much liquid or they will stew and that’s not good. If you start getting too much liquid, turn the heat up and cook some of the liquid off.
Once the mushrooms are cooked all the way through, remove and repeat with the next batch.
Oven Baked Method
Pre-heat oven to 200°C (390°F) (unless cooking with tomatoes and details with tomato cooking instructions). Remove stems and place mushrooms in baking dish top side down. Throw the stems in as well.
Add about ½ tsp vegan margarine into the underside of each mushroom. Drizzle with olive oil and put in oven. Check mushrooms after about 10 minutes to make sure that they are not dry. Cook for approx. 20 minutes (very large mushrooms may need longer).
The mushrooms and tomatoes can be cooked earlier and reheated when the risotto is ready.
Recipe by my vegan plate at https://www.myveganplate.com/herb-onion-risotto/