In a large saucepan, heat up the olive oil until hot. Add the onions and cook until translucent over medium heat. Add the garlic and cook for about a minute.
Deglaze the pan with the wine and let it cook down for a minute or two. I like to add the mushrooms at this point. They will release a bunch of water and the wine and water will continue to cook down until there is nothing left.
Add the demi glace and season with salt and pepper. Let this cook to desired thickness. Reduce heat to medium low and then add the sour cream. Stir to incorporate and continue to cook for a minute or two to heat through.
Taste and adjust seasoning with salt and pepper.
Drain the pasta and get everything ready to plate.