First we need to prep the ingredients. Finely dice the onion and slice the mushrooms thick, about 1/4 inch thick. And thinly slice the spring onion as well if you are going to use that.
Heat 1-2 tablespoons of olive oil in a non stick pan on medium heat, when hot, add the onion, cook, stirring them, until softened and starting to get golden brown.
Pour the onions and any oil, into a bowl and set that aside.
Add another 1-2 tablespoons of olive oil to the same pan, when hot, add the mushrooms.
Saute the mushrooms until starting to lightly brown, they will be soft, and release their water.
When the mushrooms are ready, drain most of the liquid in the pan, leaving just a little liquid in the pan. Add the onion and sour cream, season as desired with salt and pepper. Stir everything together and heat through then remove from heat.
Place in a serving bowl, sprinkle with spring onion, serve as a side with any meal, and enjoy.