In a large mixing bowl, sift the flour, cornstarch and salt. Cut in the butter and mix until it resembles coarse crumbs. In a small bowl, whisk the eggs and sour cream together. Stir the egg mixture into the flour mixture, stirring about 1 minute, until incorporated. This dough will be crumbly. Turn the dough onto a floured surface and sprinkle it liberally with iced water. Work the dough with a pastry scraper until the dough is moist. Dust with flour and knead a few times until smooth. Cut off 1/4 of the dough and shape into a smooth cake. Divide the remaining dough in half and shape into rectangles. Wrap all three pieces with plastic wrap and refrigerate for at least 30 minutes. Preheat the oven to 400 degrees F. Place the salmon in a saute pan, over medium-high heat and cover with the court bouillon, add water if necessary. Bring to a boil and skim away the fat. Reduce the heat to medium and simmer for 15 minutes. Remove the salmon from the liquid, reserving and set aside to cool. Remove the skin and flake the salmon. Season the salmon with salt, pepper and the paprika. Cover the salmon and refrigerate. In another saute pan, over medium-high heat, melt 3 tablespoons of the butter. Add the mushrooms and shallots. Season with salt and pepper. Saute for 2 to 3 minutes or until the mushrooms wilt and most of the liquid has evaporated. Remove from the heat and set aside. Wipe out the saute pan, and place back over the heat and melt the remaining 3 tablespoons of the butter. Add the onions. Season with salt and pepper. Saute for 2 to3 minutes or until wilted. Add the rice and continue to saute for 1 minute. Stir in the reserved court bouillon. Bring to a boil, reduce the heat to medium, cover and cook for about 15 minutes or until tender. Remove from the heat and cool completely. Stir in the sour cream. Season with salt and pepper. To assemble, on a floured surface, roll out one piece of the dough into a rectangle, (15 by 9 by 1/8-inch thick) Line a sheet pan with parchment paper. Place the rolled dough on the baking sheet. Using the tip of the knife, mark an area 12 by 7 to center your filling. Spread in layers: rice, mushrooms, 1/2 of the salmon, peppers, and remaining salmon. Roll out the other dough and place on top pressing down to form a rectangular loaf. Seal the ends of the dough completely and egg wash the entire dough. Roll out the remaining dough and cut into strips and leaves. Use these to cover seams and decorate the loaf. Brush again with the egg wash. Using a sharp knife, make a couple of cuts at the top of the loaf. Place in the refrigerator and chill for 30 minutes. Remove from the refrigerator and bake for 1 hour, brushing halfway through the baking process with the egg wash. Remove from the oven and cool for 5 minutes before serving. Serve with watercress and Hollandaise Sauce.