1. Preheat the oven to 170C. Place ricotta, parmesan and eggs in a food processor. Process until a smooth paste forms, about 20 seconds.
2. Add rocket, parsley, onion, sage and salt and pepper to taste. Blend until smooth, about 30 seconds. Spoon the cheese mixture into a well-oiled, 20cm, deep springform pan.
3. Thinly slice sweet potato lengthwise with a vegetable peeler. Arrange slices on top of the tart. Drizzle with olive oil. Bake until the cheese mixture is firm, about 1 hour. (Cover the top of the tart loosely with aluminium foil if it starts to brown too much during cooking.)4. Remove the tart from the oven. Allow to cool to room temperature before removing from the pan. Slice wedges and serve drizzled with olive oil and sprinkled with sage leaves and salt.