Cook onion and pepper in two tablespoons butter five minutes, without browning; add water and cook until onions are soft (about forty minutes). Rub through a sieve. Melt remaining butter, add flour and stir to a paste; add gradually scalded milk, stirring constantly. Combine mixtures, add seasonings. Heat to boiling point, remove from range, add yolk of egg slightly beaten. Pass Parmesan cheese and hot, crisp crackers. Two tablespoons cheese may be added to soup when adding egg yolk. Serve very hot.