Add the chopped rhubarb to a small saucepan and cover with water. The water should just cover the top of the rhubarb. Bring to a boil, then reduce heat and simmer for 20 minutes. Once the rhubarb is mushy, strain the mixture through a fine-mesh sieve to separate the liquid from the rhubarb.

Place the strained liquids back in a sauce pot; add the sugar and sprigs of thyme. Bring the liquid back to a boil, reduce heat and simmer for another 5 minutes. Let the syrup cool

Using a mixing glass and barspoon, stir the gin, rhubarb syrup and lemon juice together. Prepare 4 highball glasses with ice. Pour the gin mixture over the ice and then top with the prosecco. Garnish with a lemon twist and a ribbon of rhubarb.