Preheat oven to 350°F. Spray rimmed baking sheet with nonstick spray. Toss potatoes and 1 tablespoon oil on sheet; sprinkle with thyme, salt, and pepper. Bake until almost tender, about 12 minutes; set aside.
Maintain oven temperature. Sprinkle fish inside and out with salt and pepper. Stuff each with marjoram sprigs and lemon slices. Arrange fish in 13x9x2-inch glass baking dish. Pour in wine, 1/4 cup water, and lemon juice. Drizzle with 1 1/2 tablespoons oil.
Roast fish 15 minutes. Scatter olives and tomatoes around. Tuck in reserved potatoes and sprinkle with nuts. Continue to roast until fish is just opaque in center, about 20 minutes longer. Sprinkle with fleur de sel