Set a strainer over a heatproof bowl. In a large saucepan, heat the oil with the shallots, scallions and onion and fry over moderately high heat, stirring frequently, until the onions are browned, 18 to 20 minutes. Carefully pour the onions into the strainer
and shake off the excess oil. Spread the onions on a paper towel–lined plate to drain. Let cool for 5 minutes. Reserve the oil for another use.
In another bowl, fold the onions into the sour cream; season with salt and pepper.