Heat the butter and oil in a saucepan and gently soften the onions for about 15 minutes. Add the seasoning and sugar, and leave to cook for 30 minutes. Then add the flour and stir continuously for another few minutes. Add the boiling water and vermouth. Cover and simmer for 45 minutes. Taste and correct the seasoning. Before serving, add the brandy, Worcestershire sauce and Tabasco, then pour the soup into individual bowls. Top each serving with a piece of toast and cover with Parmesan.