Put the potatoes in a saucepan; cover with cold water by 1 inch and season with salt. Bring to a boil, then reduce the heat to medium and simmer 8 minutes. Add the lima beans; simmer until the potatoes are fork-tender, 4 minutes. Reserve 1/4 cup cooking water, then drain and return to the pot. Add the sour cream, 1 tablespoon butter and the reserved cooking water. Season with salt and pepper and smash.
Combine the paprika, thyme, cayenne and 1 teaspoon salt in a bowl; sprinkle on both sides of the salmon. Heat the vegetable oil in a large skillet over medium heat. Add the salmon and cook until browned and just cooked through, 3 to 4 minutes per side.
Stir the scallions into the potatoes and top with the remaining 1 tablespoon butter. Serve with the salmon and lime wedges.