Heat the olive oil in a large skillet over medium-high heat. Pat the scallops dry with a paper towel, add the scallops to the pan and cook for 3 minutes on each side until done. Remove the scallops from the pan and keep warm. Add the mushrooms and shallots to the pan and sauté for 3 minutes or until the liquid evaporates and the mushrooms darken. Stir in the champagne, mustard, salt and tarragon and using a wooden spoon, loosen the browned bits and mix them with the champagne and herbs. Remove the sauce from the heat and stir in the sour cream. Serve the sauce with the scallops and garnish with fresh tarragon.