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Otherwise, you can buy the shoots whole or diced at many local super markets. If they are whole, you have to slice off the spines with a sharp knife. Danger: be careful not to stab yourself! Make sure the knife is sharp and that you get all of them! You can trim about 1/4 inch all the way around to get the ones on the edges. When done, cut into 1/2 inch strips, or dice into 1/2 inch squares. (Of course, you can cheat and buy some cactus in a can already cooked! Then skip the cooking part.)
Heat some water in a medium pan and toss in the diced cactus, with some garlic cloves, onions, and a dash of salt and boil (15 minutes?) until tender. Remove the cooked cactus and place them in a strainer and wash thoroughly with cold water until all the "slime" is removed. Then place in a large bowl to cool, while you dice fresh cilantro, avocado and tomatoes. When done, stir in and mix with the cactus. You can also squirt a little lime juice into the mix. Mmmmm. Chokes me up just thinking about it!
Serve as a side dish with any food and wait for "what's this?" reactions and some unexpected compliments. Some never acquire a taste for cactus but that is to be expected. There are still deadheads around that don't like onions, garlic, spinach, artichokes or even mustard! Imagine that.