1. Thinly slice the onion and soak in cold water while you prepare the rest of the salad.
2. Smashed the garlic and sprinkle with a pinch of salt, and, with the flat side of a large knife, mash and smear the mixture to a coarse paste. Transfer to a large serving bowl. Add the lime juice, 2 teaspoons salt, and chili powder. Gradually whisk in the olive oil, starting with a few drops and then adding the rest in a steady stream to make a dressing. Toss the shrimp in the dressing.
3. Add the mango, beans, lettuce, and cilantro to the bowl. Drain and dry the onion, and scatter over the salad. Gently toss the salad together and serve.
When you're buying a mango, pick up one in each hand and see how they feel. The heavier they are, the juicier, and if they've got soft, firm skin and a sweet smell, take 'em home. If all the mangoes in the store are rock-hard, buy the heaviest one and take it home to ripen in a cool place. Look for ready-to-go shrimp done up for shrimp cocktail at the fish counter (check the deli counter, too) of your grocery store. Know How: Roll limes (or oranges, or lemons) under your hand on the countertop before you cut them in half and juice them - you get the most juice that way. We love the convenience of canned beans, but not the salty goopy stuff that clings to them. That's why we always dump them into a colander and run under cold water before using them.
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