To make the limoncello, peel the lemons removing as much white pith as possible, then put the peel into the vodka with the thyme (if using) and put in a dark place for two weeks, shaking the bottle daily.
Put the sugar in a pan with 300ml water and heat until the sugar dissolves and you’re left with a syrup.
Strain the vodka to remove the lemon peel and thyme and then pour into a sterilised bottle with the sugar syrup. Leave to infuse for a few hours or overnight.
When you’re ready to make the cocktail, put the limoncello (25ml) and gin into a glass, fill with ice, then top with soda and garnish with a sprig of thyme and a lemon slice.