Trim the leeks and cut into 1" sections. Put in boiling water for about 5-10 minutes until tender. Drain, but keep the water used for boiling. In sauce pan, melt butter. Add flour, stirring constantly. Add about 2 cups of the water. Once it is thickened, stir in the creme fraiche. Place the leeks in a buttered ovenproof dish. Cover with sauce. Add cheese on top. Bake in 350 F oven until cheese is melted, about 10 minutes.