Cut the scallions in thin slices, separating the white bulb from the tender greens. Heat the butter in a skillet, add the white part of the scallions and the mushrooms, and toss and cook over medium heat 5 minutes. Pour in the vermouth and let boil a few seconds, add half the tarragon, the crabmeat, and the scallion greens, stirring.
When hot, stir in the sour cream and/or yogurt, and salt and pepper to taste. Cook just long enough to heat through. Divide evenly over hot buttered toast, and scatter the remaining tarragon on top.