These chicken tostadas feature boneless, skinless chicken breasts in a bold and flavorful way: marinated and served alongside grilled, tangy tomatillos. Instead of frying tortillas for the base, we charred them on the grill for a slightly crisp but smoky flavor.
Combine the lime juice, garlic and chipotle chile in a medium nonreactive bowl; season with salt. Add the chicken, toss well to coat, and refrigerate 2 hours. Remove the chicken from the marinade; thread onto 4 (12-inch) skewers lightly misted with cooking spray.
Preheat the grill to medium. Mist the tortillas with cooking spray. Add the tortillas to the grill; cook, turning once, until lightly toasted, about 1 to 2 minutes per tortilla.
Meanwhile, add the chicken and the whole tomatillos to the grill; cook, turning often, until tomatillos are charred and chicken is cooked through, 4 to 6 minutes. Remove the skewers from the chicken.
Coarsely chop the tomatillos and toss with oil and salt to taste. Place toasted tortillas on plates; generously top with romaine, tomatillos, chicken, and queso fresco. Serve with onion, cilantro sprigs, and a lime wedge if desired.