This is how I cook white mushrooms (1 pack equal to 8 oz or around 200 gr). The same recipe can also be used for cremini mushrooms.
Starting point is to wash them well. Use brush if needed to remove dirt. Slice mushrooms into a medium sized bowl. Slice 1 large peeled onion.
In a frying pan heat 1 Tbsp of cooking oil. I mostly use grapeseed oil for cooking and olive oil for salads.
Saute onions with a pinch of salt until golden and soft. Add sliced mushrooms and stir over medium heat until all liquids released by mushrooms evaporate. This will take approximately 10 minutes.
Serve mushrooms with a side of garlic-flavored mashed potatoes or whatever you choose. Brown rice, quinoa, white rice, buckwheat, couscous are great with mushrooms as well. Garnish with herbs and olives (optional).