Prepare the topping while the pizza dough is rising. Cut each onion into eight wedges, place them in a shallow roasting dish and drizzle over the oil. Top with half of the sage and season well with salt and pepper. Roast in a preheated oven, 220°C (425°F), Gas Mark 7, for 25–30 minutes until soft and caramelized. Add the vinegar and cook for a further 5 minutes. Leave to cool.
Increase the oven temperature to 240°C (475°F), Gas Mark 9 and place a large baking sheet on the middle shelf. Cream together the Gorgonzola and crème fraîche.
Roll out one piece of pizza dough on a floured surface to form a thin 23 cm (9 inch) round. Transfer to a well floured board or second baking sheet and top with a quarter each of the onions, the cheese mixture, the remaining sage and walnuts. Season with pepper and drizzle with a little olive oil.
Slide the pizza on to the heated baking sheet. Bake for 10–12 minutes until the base is crisp and the topping melted. Prepare the second pizza while the first one is cooking and so on with all four.
Serve the pizzas garnished with a little rocket, if liked.