How to Make It
Preheat the oven to 350°. On a large rimmed baking sheet, toss the bread with 1/4 cup of the olive oil. Toast for 15 minutes, or until lightly golden around the edges. Transfer the bread to a large bowl.
In a small bowl, whisk the mustard with the vinegar and the remaining 6 tablespoons of olive oil.
In a large skillet, melt the butter. Add the onion and cook over moderate heat until softened, about 4 minutes. Add the kielbasa and cook until it begins to brown, about 5 minutes. Stir in the sauerkraut and cook until warm. Add the sausage and sauerkraut to the bread cubes, pour the dressing on top and toss well. Season with pepper and serve.