Shell and, if necessary, devein the shrimp. Cook 5 mintues in the beer with the onion, salt, lemon slices, bay leaf and parsley. Let cool in cooking liquid, then drain.
Melt the butter over low heat and blend in the flour and sugar. Cook and stir until smooth. Add the shrimp and the tomato sauce. Cook until thick, stirring continuously.
Place on a serving platter. Garnish with decorative raw vegetables of your choice.