For Sauce: Combine shallots and vinegar in heavy small saucepan. Boil until most of liquid evaporates, about 4 minutes. Add wine; boil until most of liquid evaporates, about 3 minutes. Whisk in cream. Set sauce aside.
For Halibut: Preheat oven to 400. Heat 2 tablespoons oil in heavy large skillet over medium-high heat. Add mushrooms and crushed red pepper. Cook until mushrooms are deep golden brown, stirring frequently, about 12 minutes. Add zucchini and saute 2 minutes. Season to taste with salt and pepper.
Meanwhile, lightly oil baking sheet. Place halibut on sheet and brush with remaining 1 tablespoon oil. Bake until fish is opaque in center, about 10 minutes.
Bring sauce to simmer. Reduce heat; add butter and whisk just until melted. Remove from heat. Mix in tomatoes and sorrel. Season with salt and pepper.
Spoon Garlic Mashed Potatoes onto center of 4 plates. Place Halibut atop potatoes. Arrange mushroom mixture around potatoes. Spoon sauce over fish and serve.