Melt butter over low heat, add leek, onion & garlic & cook, covered w/buttered wax paper & lid, stir occasionally 15-20min, til onion softens. Add stock & potato, simmer, covered 15min, til potato is tender, & puree in blender/processor. Return puree to pan, stir in cream, dried sage, salt & white pepper to taste & reheat over low heat. put in bowls & top w/port, fresh sage & Stilton.