Heat a saute pan over medium heat. Coat the bottom with just enough oil to cover. Add the onions, season the with salt and pepper. Cook them slowly until they are a rich brown and very tender. Add the stock and let it simmer away until it is reduced to about a tablespoon. Remove it from the heat and let it cool.
Combine the cooled onion soup with the sour cream and stir to combine. Taste it and adjust the seasoning as necessary. Serve.