Recipe for a perfect Tosca: Start with your base, a sympathetic production which places you in Rome in 1800. Spread with a lush filling of sweeping, colourful orchestral playing. Now add the main layer of your cake: a generous portion of three star singers, all of them fully committed to acting as well as singing their roles. Finally, garnish with spectacular set pieces in each act. Over the years, the Royal Opera has proved adept at following the recipe.