Preheat oven to 400 degrees F. Line a baking sheet with parchment paper; set aside. In a large skillet heat oil over medium heat. Add onion; cook about 5 minutes or until tender. Add spinach and garlic; cook until heated through and any excess moisture evaporates. Season to taste with salt and pepper. Let cool slightly; stir in Parmesan cheese.
On a lightly floured surface, roll the pizza dough into a 16x10-inch rectangle. Evenly spread the spinach mixture over the dough rectangle. Beginning at a long side, roll up the dough into a spiral to enclose the filling; pinch the seam closed with your fingers. Slice into eight 2-inch-wide pinwheels. Arrange pinwheels, cut sides up, on the prepared baking sheet, spacing them about 2 inches apart.
Bake for 20 to 25 minutes or until golden brown. Serve warm with salsa.