Salmon with Beurre Rouge and Smoked-Salmon-Stuffed Baked Potato

Bring wine, shallots, and vinegar to boil in heavy small saucepan. Reduce heat to low and simmer until liquid is reduced to 2 tablespoons, about 5 minutes. Remove pan from heat and cover.
Do ahead: Can be made 4 hours ahead. Let stand at room temperature.
Preheat oven to 400°F. Place each potato on separate sheet of foil, sprinkle with sea salt, and wrap tightly. Bake until tender, about 1 hour. Remove foil; cool to just warm, about 20 minutes. Cut lengthwise in half. Scoop flesh into medium microwave-safe bowl, leaving 1/4-inch-thick potato shell. Add 2 tablespoons butter and milk to potatoes in bowl; mash well. Stir in sour cream and 3 tablespoons green onion tops. Season to taste with salt and pepper.
Do ahead: Can be made 2 hours ahead. Loosely cover shells and filling separately and let stand at room temperature.
Rewarm potato shells, then filling, in microwave at 20-second intervals until heated through. Stir smoked salmon into filling; season with sea salt and pepper. Mound filling in shells.
Place 1/4 cup water and salmon fillets in heavy large skillet; sprinkle with salt and pepper. Cover; steam over medium-high heat until salmon is just opaque in center, about 3 minutes.
Meanwhile, rewarm wine-shallot reduction over medium-low heat. Add remaining 8 tablespoons butter, 1 tablespoon at a time, whisking until each is melted before adding next; continue whisking until beurre rouge is thick (do not overheat or sauce may separate). Season with salt and pepper.
Spoon sauce onto 4 plates. Place potatoes on 1 side; sprinkle with remaining green onions. Arrange 2 salmon strips atop sauce on each plate; sprinkle with peppercorns and chives.