Scrub and dry the potatoes. Cut a small deep slit in the top of each. Set the potatoes on the middle rack of a preheated 375øF. oven and bake about 1 hour, or until potatoes are tender when pricked with a fork.
Let potatoes cool slightly, cut off and discard the tops, and scrape the potato pulp into a bowl. Do not scrape so deeply that you tear the potato skin. Salt and pepper the potato shells; reserve. Mash the potato pulp; reserve.
Melt the butter in a small skillet and saute the chopped onion, covered, until tender and lightly colored, about 25 minutes. Add the mushrooms and saute for another 5 minutes. Stir in the lobster or crab. Season with salt and pepper, add the vermouth, then raise heat to a boil. Stir frequently over high heat until all liquid has boiled away. Stir in creme fraiche and remove from heat.
Combine lobster mixture with the reserved mashed potato pulp and 1/2 cup Jarlsberg. Taste, correct seasoning; add heavy cream if the mixture seems too dry.
Stuff the mixture into the reserved potato skins; mound the filling slightly. Sprinkle additional grated cheese on top and place on baking sheet.
Bake again at 400F., until potatoes are hot and cheese is bubbling. Serve immediately