Paddles from the nopal cactus, callednopales, make a delicious taco filling.
If cactus paddles still have thorns, grasp with a gloved hand or pair of tongs and shave with a sharp knife (be sure to peel just the thorns). Rinse under cold water.
To cut nopales, hold paddle in your hand and shave off 2-inch-long slivers with a knife, turning paddle every few cuts so that pieces remain uniform.
Bring a large pot of salted water to a boil. Add nopales and half of the chopped onions. Boil briskly for 10 to 15 minutes or until tender. Drain and rinse in 2 changes of cold water.
In a large saute pan over medium heat, cook remaining onion with the garlic in the oil until soft, about 5 minutes. Add nopales, tomatoes, and chiles, season with salt, and cook for about 10 minutes. Sprinkle in cilantro and remove from heat. Serve hot or at room temperature.