Dice the onion, carrots and leek and add to a large casserole dish with the olive oil. Soften for 6-7 minutes, then stir in the flour. Gradually add the stock, then bring to the boil. Simmer for 10 minutes, or until the carrots have softened.
Stir in the sweetcorn, crab meat and creme fraiche. Bring back to a simmer, taste and season with salt and pepper.