Finely chop your onions. Slice mushrooms into 1/4″ thick slices.
In a medium, non-stick pan, saute onions in 1-2 tbsp olive oil for a few minutes or until golden.
In a separate, large pan, sauté sliced mushrooms in 1-2 tbsp olive oil until soft.If mushrooms release a lot of juice, drain excess juice; leaving some juices behind. Baby Bella mushrooms don’t seem to get as juicy as buttons or cremini so we didn’t have to drain.
Combine mushrooms and onions. Add in 3 tsp sour cream and salt and pepper to taste.