Preheat the oven to 250 degrees F. Bring a large saucepan of salted water to a boil; add the potatoes and cook until just tender, about 5 minutes. Drain and pat dry.
Meanwhile, melt 1 tablespoon butter in a large skillet over medium heat. Add the bratwurst and cook, turning, until browned, about 4 minutes. Remove to a rimmed baking sheet and keep warm in the oven.
Add 1 tablespoon butter to the skillet along with the potatoes and apples; toss. Season with 3/4 teaspoon salt and a few grinds of pepper. Increase the heat to medium high and cook, tossing occasionally, until the potatoes and apples are golden brown and tender, 8 to 10 minutes. Add the scallions and sage and cook until the scallions are wilted, about 2 minutes. Add the vinegar, honey and the remaining 1 tablespoon butter. Cook, stirring, until the potatoes and apples are glazed, about 1 more minute.
Mix the sour cream with the mustard in a small bowl. Serve the bratwurst with the potatoes and apples, sauerkraut and sour cream mixture.