Place the water in a medium saucepan over high heat.
When the water begins to boil, add the vinegar and reduce the heat so that the water is slowly simmering.
Add the scallops and poach for 8 minutes, then remove the scallops to a paper towel to cool.
Increase the heat and simmer the poaching liquid for about 20 minutes, or until it is reduced to about 2/3 cup.
While the poaching water is simmering, place the olive oil in a small skillet over medium high heat.
Add the shallot and sauté for about 2 minutes. Stir frequently.
Add the mushrooms and cook for about 10 minutes. Stir frequently.
When the mushrooms are cooked, but not dried out, add the tarragon, sherry, and 1/16 teaspoon of the salt.
Stir and simmer for another 2 minutes.
Divide the mushroom mixture between two shallow oven proof dishes (such as a shallow soufflé or au gratin dish).
Top the mushroom mixture with the cooked scallops and set aside.
Preheat the oven to 325°F.
Wipe the pan used for the mushroom mixture clean and place over medium high heat.
Add the butter, and when it is melted, add the flour. Whisk continuously for about 2 minutes.
Slowly add the reduced poaching liquid from the scallops. Whisk continuously to create a smooth, thick sauce.
Slowly add the milk while whisking continuously.
Turn off the heat, add the goat cheese, and whisk until smooth.
Divide the sauce evenly over the top of the scallops.
Sprinkle the bread crumbs and then the gruyere cheese evenly over the top of the scallops and put the dishes in the oven.
Bake for 8 to 10 minutes until the sauce begins to bubble and the cheese is melted.
Serve.