Preheat oven to 220°C. Heat a barbecue grill or chargrill on high. Spray the pumpkin and onion with olive oil spray. Cook the pumpkin and onion on the grill, in batches, for 1 min each side or until lightly charred and just tender. Transfer to a plate.
Meanwhile, heat a large frying pan over medium-high heat. Add the sausage and cook, stirring with a wooden spoon to break into lumps, for 5 mins or until browned.
To make the rocket pesto, process the rocket, basil, parmesan, pine nuts and garlic in a food processor until finely chopped. With the motor running, add the oil in a thin, steady stream until well combined. Season.
Place pizza bases on 2 baking trays. Spread evenly with half the rocket pesto. Sprinkle with half the mozzarella. Arrange pumpkin, onion, sausage and half the sliced fennel on top. Sprinkle with the remaining mozzarella. Bake, swapping the trays halfway through cooking, for 15 mins or until mozzarella melts and the bases are golden brown.
Combine the rocket, remaining sliced fennel and reserved fennel fronds in a bowl. Drizzle with oil and vinegar and toss to combine.
Top pizzas with fennel mixture and drizzle with the remaining pesto.