To make pickled onions, combine the thinly sliced red onion and the red wine vinegar in a small bowl. Cover; chill for at least one hour.
Pour the orange juice into a large salad bowl. Whisk in the olive oil and orange zest; season with 1/4 teaspoon salt, and a pinch of black pepper. Toss the cantaloupe, watermelon, and mango in the dressing. Just before serving, stir in the pickled onions and cilantro leaves.
Cut the avocados into 1/2 inch dice; place in a small bowl. Sprinkle with the lime juice, and season with the remaining 1/4 teaspoon salt. Stir lightly to combine. Spoon an even amount of avocado into the bottom of 6 large martini or wine glasses. Top avocado with melon salad; garnish each salad with a lime slice.