Prep Glass: Fill your Old Fashioned glass with ice cubes. Give it a quick stir and set it aside.
Muddle Raspberries: In a mixing glass, muddle 3 fresh raspberries until they release their juices.
Add Campari: Pour 1 oz Campari into the mixing glass.
Add Sloe Gin: Add 1 oz Sloe Gin.
Add London Dry Gin: Measure and pour 1½ oz London Dry Gin into the mixing glass as well.
Add Ice and Stir: Fill the mixing glass with ice. Using a bar spoon or a long stirrer, stir well (for about 15-20 seconds). Give it a taste and stir a bit more if you feel like more dilution is necessary.
Strain: Discard the ice in the Old Fashioned glass and refill it with fresh, new ice cubes. Then, using both the cocktail strainer and fine strainer, strain the drink into the ice-filled glass.
Garnish: Garnish with a fresh raspberry and serve immediately.