Heat stock in a saucepan until simmering, then maintain at this heat.
Heat olive oil in a large pan and fry onion over medium heat until soft but not coloured.
Add the rice and stir over a high heat until grains are well coated in oil and warmed through. Add white wine and stir until most of the liquid has been absorbed. Reduce heat to medium, add stock, a ladleful at a time, stirring constantly and allowing each ladleful to be absorbed before adding the next.
Continue until rice is tender, with a slight bite, and has a creamy consistency (about 20 minutes).
Add a final ladle or 2 of stock, preserved lemon, crabmeat, butter, parmesan (if using), salt (remembering that the butter, crab and preserved lemon are all salty) and pepper and beat vigorously with a wooden spoon until all the butter is incorporated and the crab has broken up into thin wisps. The finished risotto should be quite soupy, the Venetians call it all'onde (wave-like).
Taste, add extra salt if needed, stir through chives and serve immediately.
Notes: You can use other types of crabs for this recipe, or 300g fresh crabmeat if you don't want to pick your own. Vialone nano is the preferred rice in the Veneto, where this soupy style of risotto is most popular, use arborio or carnaroli rice if it isn't available.