For the soup:
1) Preheat the oven to 200°C.
2) Combine the potatoes and leeks on a baking tray in a single layer. Add the olive oil, one teaspoon of salt, and half a teaspoon of pepper and toss to coat the vegetables evenly.
3) Roast for 40 to 45 minutes, turning them with a spatula a few times during cooking, until very tender.
4) Add the rocket and toss to combine. Roast for four to five more minutes, until the rocket is wilted.
5) Remove the pan from the oven and place over two burners. Stir in the wine and 250ml of the chicken stock and cook over low heat, scraping up any crispy roasted bits sticking to the pan.
6) In batches, transfer the roasted vegetables to a food processor fitted with the steel blade, adding the pan liquid and about 1.25 litres of the chicken stock to make a puree. Pour the puree into a large pot or casserole dish. Continue to puree the vegetables in batches until they're all done and combined in the large pot.
7) Add enough of the remaining 250 to 500ml of stock to make a thick soup. Add the cream, creme fraiche, two teaspoons of salt, and one teaspoon of pepper and check the seasonings.
8) When ready to serve, reheat the soup gently and whisk in 30ml of white wine and 25g of Parmesan cheese.
9) Serve hot with an extra grating of Parmesan and crispy shallots, if using.
For the crispy shallots:
1) Heat the oil and butter in a saucepan over medium-low heat until it reaches 105°C on a thermometer.
2) Reduce the heat to low, add the shallots, and cook for 30 to 40 minutes, until they are a rich golden brown. The temperature should stay below 126°C. Stir the shallots occasionally to make sure they brown evenly.
3) Remove them from the oil with a slotted spoon, drain well, and spread out to cool on paper towels.
4) Once they have dried and crisped, they can be stored at room temperature, covered, for several days.