Cut the fennel, peeled celery and leek in fine julliene. Add butter in a pan as well as the vegetables. Allow them to sweat by adding a bit of water. Cover the pan with a lid or baking paper. Turn the heat low and check if it is not burning. Stir once in a while. When cooked, season with salt and pepper and allow to cool down. When cooled down, mix in fresh cheese as well as the egg white.