Prepare the fruit: Wear protective gloves and handle unpeeled prickly pears with kitchen tongs. If harvesting, use tongs to gently detach ripe, undamaged fruit from the cactus.
Remove the spines: Using long-handled metal tongs, carefully rotate each fruit over a gas flame to singe the spines. Keep your hands and gloves away from the flame.
Wash and peel: Rinse thoroughly under running water, gently scrubbing with a vegetable brush. Continue using gloves and tongs, as tiny spines may remain. Trim the ends, make a shallow lengthwise cut through the skin, and carefully peel, preserving as much flesh as possible.
Cook the fruit: Place the peeled prickly pears in a large pot and add the water. Bring to a boil over medium heat, then reduce the heat to low. Gently crush the fruit with a potato masher and simmer for 20 minutes, stirring occasionally.
Strain: Remove from the heat. Carefully pour the mixture through a fine-mesh strainer lined with several layers of closely woven cheesecloth into a heatproof bowl. Let drain, gently pressing with a spoon to extract the juice. Discard the seeds and pulp.
Add the sugar: Return the strained juice to the pot. Add the sugar and heat gently over low heat, stirring until completely dissolved.
Add the lemon: Stir in the strained juice of one lemon. Cook gently for 2 minutes, then remove from the heat.
Cool and bottle: Cool promptly, using an ice-water bath around the pot to speed cooling. Strain the cooled syrup again through clean, closely woven cheesecloth. Pour into clean bottles, close with clean lids, and refrigerate.
Serve: Mix a small amount of prickly pear syrup with cold water. Stir, taste, and add more syrup or water until the sweetness suits you. Serve chilled, with ice if desired.
