Lightly fry a thinly sliced onion in butter for a few minutes in a saucepan. Add a small, sliced cucumber, stir for 5 minutes, and add a vegetable stock cube and about 3 cups of water to cover the vegetable. Cook for 10 minutes, and then stir in Boursin (without letting the Boursin melt). Leave to cool in the fridge for 3 hours. Serve the chilled soup in small glasses garnished with crabmeat