prepare a gas or charcoal fire for direct cooking over medium-high heat, 500°.
Grill the thyme sprigs until fragrant and lightly charred, about 15 seconds.
In a mixing glass or cocktail shaker gently mash the grilled thyme with the lime juice with another or similar tool like the handle of a wooden spoon.
Add the gin and Limoncello and fill the shaker with ice. Stir well. Strain into a chilled rocks glass filled with fresh ice, garnish with thyme, and serve. Serves one.