Preheat a grill or grill pan over medium-high heat. Toss the plums, nectarines, onions and jalapenos with 1 tablespoon of the oil and 1/2 teaspoon salt in a bowl. Toss the chicken with the chipotle powder, oregano, 3/4 teaspoon salt and the remaining tablespoon oil.
Grill the fruit mixture, turning occasionally, until the fruit is slightly softened and charred in places and the onions are slightly charred and crisp-tender, 5 to 7 minutes. Transfer to a bowl. Grill the chicken, turning once, until just cooked through, 7 to 8 minutes per side, and then transfer to a cutting board.
Place the jalapeno and half of the onions in a blender with 2 tablespoons water and blend until smooth. Add about two-thirds of the fruit to the blender and pulse until slightly chunky. Transfer the salsa to a bowl and stir in the cilantro.
Chop the chicken and the remaining fruit and onions into bite-size pieces. Serve the chicken and fruit mixture in the tortillas topped with the salsa, shredded lettuce and cilantro sprigs.
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