Finely chop your onions. Cut mushrooms into 1/4" thick slices.
In a medium, non-stick pan, saute onions in 1-2 Tbsp olive oil for a few min or until golden
In a separate, large pan, sauté sliced mushrooms in 1-2 Tbsp olive oil until soft. Drain off excess juice if needed; leaving behind a small amount of juice.
Combine mushrooms and onions. Add in 3 Tbsp sour cream and sprinkle with salt and pepper to taste.