Preparation Melt butter in heavy large skillet over medium-low heat. Add shallots and sauté until beginning to soften, about 3 minutes. Add watercress and stir until wilted and still bright green, about 3 minutes. Add cream. Increase heat to high and bring to boil. Remove from heat. Puree hot sauce in blender until almost smooth. Transfer to heavy small saucepan. Season with salt and pepper. (Can be made 8 hours ahead. Refrigerate.) Butter 2 steamer racks and place in 2 large saucepans over simmering water. Season salmon fillets with salt and pepper. Place salmon, skin side down, on steamer racks. Cover saucepans and steam until salmon is just opaque in center, about 10 minutes. Whisk sauce over low heat to rewarm. Transfer salmon to platter. Spoon some of sauce over salmon. Garnish with additional watercress. Serve, passing remaining sauce separately.
Preparation Melt butter in heavy large skillet over medium-low heat. Add shallots and sauté until beginning to soften, about 3 minutes. Add watercress and stir until wilted and still bright green, about 3 minutes. Add cream. Increase heat to high and bring to boil. Remove from heat. Puree hot sauce in blender until almost smooth. Transfer to heavy small saucepan. Season with salt and pepper. (Can be made 8 hours ahead. Refrigerate.) Butter 2 steamer racks and place in 2 large saucepans over simmering water. Season salmon fillets with salt and pepper. Place salmon, skin side down, on steamer racks. Cover saucepans and steam until salmon is just opaque in center, about 10 minutes. Whisk sauce over low heat to rewarm. Transfer salmon to platter. Spoon some of sauce over salmon. Garnish with additional watercress. Serve, passing remaining sauce separately.