Pre-heat the oven to the suggested time on the pizza shell packaging or, for homemade, see notes.
Brush the olive oil all over the pizza crust. Top the pizza with the caramelized onions, followed by the cheese, apple slices, and walnuts. You can sprinkle a little extra cheese on top at this point if desired. (I always do; cheese is my weakness.)
Place the pizza in the oven. If store-bought, cook as per the package's directions. If homemade, cooks as per the recipe's directions. You're looking for a golden crust and melted cheese.
Remove the pizza from the oven and let it cool for 5 minutes. Once slightly cooled, drizzle balsamic over it, sliced and serve.