Step 1Place the prickly pears in a medium saucepan and cover halfway with water. Place the pan over high heat and bring the water to a boil. Cook the pears, swirling and rolling them in the water occasionally, until very soft and falling apart, about 10 minutes. Using a potato masher, mash the pears in the pan, then scrape the pulp and liquid through a fine strainer into a bowl; discard the skin, pulp and seeds. Rinse out the pan.
Step 2Pour the juice back into the saucepan (you should have about 1 ½ cups), then add the sugar. Place the pan over high heat, stirring to dissolve the sugar, then boil for 5 minutes, using a spoon to skim off any scum that rises to the surface.
Step 3Pour the syrup into a bowl, let cool, then transfer to a storage container and refrigerate until ready to use, up to 2 weeks.