Dip the herring fillets into the flour, coating them well on both sides. Then shake off the excess flour. In a heavy 10- to 12-inch skillet, heat the oil over high heat until a light haze forms above it. Droop in the herring and fry about 2 minutes on each side, turning them over carefully with a large spatula. When they are golden brown on both sides, transfer them to a heated serving plate and cover them loosely with foil to keep them warm while you make the sauce.
2 tablespoons of butter
In the same skillet, melt 2 tablespoons of butter over moderate heat. Add the chopped onions and, stirring frequently, cook for about 5 minutes, or until they are soft but not brown. Stir in the flour, then the mustard, sour cream, salt and a few grindings of black pepper. Stirring constantly, simmer over low heat for about a minute, or until the sauce thickens lightly. Off the heat, stir in the lemon juice and taste for seasoning. Pour the sauce into a small bowl and serve at once with the herring as a first course or as part of a zakuska table.