Cut the limes in half lengthways. Then hollow out the lime halves by carefully running a knife around the inside edge of the skin and using a spoon to remove the juice and flesh into a bowl. Use your fingers to remove any remaining flesh. Make sure you keep the juice because you will need this for later in the recipe.
Take a muffin tray and place each lime half into a muffin mould to keep them upright.
Prepare the jelly mixture by pouring the tonic water and sugar into a pan on a medium heat and sprinkle the agar flakes on top. Bring the mixture to the boil, then simmer for 2-3 minutes. Only stir the mixture once the agar flakes have dissolved.
Remove the pan from the heat and transfer the mixture to a measuring jug. Add the gin, then add 2 tbsp of lime juice from earlier. Stir to combine.
Pour the liquid evenly into the lime halves.
Transfer the muffin tray to a refrigerator and leave to set for at least 2 hours.
Remove the muffin tray from the refrigerator and using a sharp knife cut the limes in half to create quarters.