Cut the watermelon and scoop out the pink flesh, saving it for eating. Use a vegetable peeler or knife to remove the green skin from the rind, leaving only the firm white layer.

Cut the white rind into 2.5 cm / 1-inch cubes. You should have about 800 g of rind pieces.

Put the cubes in a bowl or in your 6-quart / 5.7 L stockpot and cover with cold water. Soak for 8 to 12 hours, or overnight in the fridge, so the pieces stay firm.

Drain the rind and cover with fresh water in the same pot. Bring to a boil and simmer for 12 to 15 minutes, until a knife slides through easily but the pieces still hold their shape. Drain well.

In a 3-quart / 2.8 L saucepan, combine the sugar and 400 ml water. Bring to a boil over medium heat, stirring until the sugar fully dissolves.

Add the drained rind, lemon juice, lemon slices, cinnamon stick, and cloves. Lower the heat to a gentle simmer.

Simmer for 45 to 55 minutes, stirring occasionally and skimming off any foam, until the rind turns translucent amber and the syrup thickens enough to coat the back of a spoon. The syrup should read about 104C/219F on a candy thermometer if you have one.

Remove the cinnamon stick and cloves. Let the pot cool for 20 minutes.

Spoon the rind and syrup into sterilized jars, leaving a little headspace at the top. Seal, cool completely, then refrigerate.