In a large saucepan, bring the potatoes and enough cold water to cover to a boil. Add the carrots, lower the heat and simmer until crisp-tender, 10 minutes. Using tongs, transfer the carrots to a plate. Cook the potatoes until tender, 6 to 10 minutes more. Drain, then return the potatoes to the pot, toss with the butter and season with salt and pepper. Keep warm.
Meanwhile, in a large, deep, heavy skillet, heat 1 tablespoon olive oil over medium heat. Add the sausages and fry until browned, 6 to 8 minutes. Transfer to a platter and tent with foil.
Wipe the skillet and add the remaining 1 tablespoon olive oil; add the scallion whites and cook over medium-high heat for 1 minute. Add 1 cup water, the sauerkraut, bay leaves, reserved sausages and carrots and heat through, 1 to 2 minutes. Stir in the scallion greens; discard the bay leaves.
Serve the sausages and sauerkraut with the potatoes, mustard and sour cream alongside.