Cut the lemon in half. Set one half aside and slice the other half into 1cm slices, discarding the final end slice. Repeat with the lime. Lightly oil the grill and barbecue the lemon and lime slices over medium heat until the first side is seared with blackened grill marks (about 3 minutes), then flip and cook the other side (about 2 minutes). Set aside

Chill the glasses by filling with ice while you make the cocktails

Mix the honey with a splash of hot water to loosen and add to a cocktail shaker. Roughly chop the citrus slices and add to the cocktail shaker with the tequila. Squeeze the reserved lemon and lime halves into the shaker

Using a cocktail muddler, crush and mix the fruit until they are thoroughly pressed and mixed. Fill the shaker about three-quarters with ice. Put on the lid and shake until the shaker is very cold and frosted

Remove the ice from the glasses and pour the liquid through a fine meshed strainer into the chilled glasses. As there is a lot of debris in the mixture, particularly the charred parts of the citrus slices, it’s important to use a fine mesh strainer when adding the liquid to the glass

Add a splash of tonic water (if using) before garnishing with half of the thin lemon slice