Pretzel salt baked into the crust's edges, plus sliced kielbasa and green apple in the topping, put this tart squarely on German ground.
Preheat oven to 450 degrees with rack in lower third. On a parchment-lined baking sheet lightly dusted with flour, stretch dough into a 10-by-15-inch rectangle, with a 1-inch border of thicker dough. Brush entire surface inside border evenly with mustard. Brush border with egg, then sprinkle generously with salt. Sprinkle cheese over mustard. Scatter kielbasa, apple, onion, and sage evenly over cheese, then drizzle with oil.
Bake until crust is deep golden brown and bottom is set, about 25 minutes; tent with foil if edges brown too quickly. Serve hot.
Buy a pound of fresh pizza dough from a pizzeria, or look for it in the freezer section of the grocery store and thaw according to the package instructions.