Fry prosciutto in the olive oil in a medium skillet over medium heat until prosciutto darkens and crisps up, about 5 minutes. Drain prosciutto and set aside; reserve drippings.
Toss basil, salad, onion, reserved oil, half the prosciutto, and a sprinkling of salt and pepper; toss to coat. Squeeze in a couple of teaspoons of lime juice to taste; toss to coat again. Taste and adjust seasonings, adding salt, pepper, and lime juice as needed.
Sprinkle remaining prosciutto over each watermelon slab, top with a portion of the salad, and serve immediately.
Recipe by Three Many Cooks at https://threemanycooks.com/recipes/salads-and-sides/basil-prosciutto-salad-on-watermelon-plates/