One by one, clean the cactus paddles: Holding a paddle with a pair of tongs, trim off the edge that outlines the paddle, including the blunt end where the paddle was severed from the plant, then slice or scrape off the spiny nodes from both sides. Cut the paddles into ½-inch squares or into ¼-inch strips (if any are longer than about 2 ½ inches, cut them in half). Scoop the cactus into a medium (2- to 3-quart) saucepan and add a little oil, a good sprinkling of salt and some sliced onion. (For a pound of paddles, you’ll need about 1 tablespoon oil, ½ teaspoon salt and a small sliced onion.) Cover and set over medium heat and cook, stirring once or twice for about 5 minutes. Uncover the pan and continue to cook, stirring regularly until all of the liquid has evaporated and the mixture is just beginning to sizzle in the oil, about 5 minutes more. Spread onto a large plate to cool.