Place a large ice cube in a rocks glass (Old Fashioned glass) to begin chilling.

In a mixing glass, measure and combine the London Dry Gin, Campari, and sweet vermouth.

Fill the mixing glass about 3/4 full with standard ice cubes.

Using a long bar spoon, stir smoothly and continuously for 20 to 30 seconds to chill and properly dilute the cocktail.

Using a strainer, pour the chilled cocktail over the large ice cube in your prepared rocks glass.

Garnish by sliding a thick fresh orange wheel down the inside edge of the glass, or by expressing the oils from an orange peel over the surface and dropping it in. Serve immediately.