In a mixing glass, combine the gin and Dubonnet, stir well. Strain into a rocks glass. Add a lemon slice, and on top of the lemon slice place a large ice cube or a couple normal-sized ice cubes.
Reportedly the daily tipple of Queen Elizabeth. Supposedly her preference is 30-70 gin to Dubonnet (so, 3/4 oz and 2 1/4 oz, rather than 1 oz and 2 oz).