Thinly slice 1/2 small red onion and soak in ice water, 10 minutes. Whisk 1/4 cup olive oil with the juice of 1 lime and 1/2 orange in a large bowl. Add a 15-ounce can hearts of palm (drained and sliced), 1 sliced mango and 1 sliced jalapeno. Drain the onion; add to the salad along with 1 bunch trimmed watercress. Season with salt and toss.