Place the chopped cactus in a 2 qt (2 lt.) saucepan, and cover with water. Bring to a boil over medium heat, cover and simmer until the cactus changes color from bright to dull green, about 10 minutes. The water in the saucepan will become gooey as the cactus cooks. Once the cactus is tender and darker in color, drain the water in the colander, and rinse the cactus well. Allow to cool.

Meanwhile, in a separate 2 qt. (2 lt.) saucepan, fill halfway full with water, and add the dried chiles. Bring to a boil over medium heat, cover and allow to simmer until tender, about 10 minutes. Remove the chiles from the water and discard the water. Remove the stems, and rinse away any remaining seeds.

Place the prepared chiles in the container of a blender or immersion blender along with the water and garlic. Blend until smooth. If needed, you can add another 1/2 cup (120ml) of water to facilitate blending.

In a small saucepan, heat the olive oil. Add the chopped onions, and sauté over medium heat until the onions are translucent, about 3 minutes. Carefully add the chile puree, stirring to combine well. Add the prepared cactus, stirring to coat the cactus well with the chile sauce. Season with salt and pepper to taste. Simmer for 3-5 minutes, until the sauce has reduced and the cactus is heated throughout.