Preheat oven to 450 degrees with the oven rack in the lower-third of oven (*If using a pre-packaged pizza dough, feel free to follow temperature on package). On a parchment lined baking sheet lightly dusted with flour, stretch dough into a 10x15-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 center is set, about 20 minute; tent with foil if edges brown too quickly. Serve hot.
Notes
For a quick pizza crust, check your local grocer or even pizzeria for fresh dough. You may also find it in the freezer section of some grocery stores. If all else fails, and you don't want to make your own, grab a tube of pre-made dough, like Pillsbury (that's actually what I did!)