Remove casings from sausage. Cook sausage in a large nonstick skillet over medium-high heat until sausage is browned, stirring until it crumbles. Remove from skillet; set aside.
Wipe skillet clean with paper towels. Cook fennel and leek in hot oil, stirring constantly in skillet over medium heat 8 minutes or until vegetables are lightly browned.
Place bread shell on an ungreased baking sheet or pizza pan. Spread pizza sauce evenly over crust. Top with cooked vegetables, sausage, onion slices, and cheese.
Bake at 450° for 10 minutes or until bubbly. Sprinkle with chopped fennel leaves, if desired.