1. In a medium bowl, whisk the lemon juice, olive oil, curry powder, smoked paprika, salt, garlic powder, and cayenne. Briefly set aside.
2. One at a time, place each chicken breast between two pieces of plastic wrap. Using the bottom of a skillet, pound the chicken to a ¼-inch thickness. (Be careful not to over pound the chicken—you don’t want to tear the meat!)
3. Place the pounded chicken breasts in a large Ziploc food storage bag, and add the marinade. Seal the bag, removing as much air as possible, and gently squish the chicken around so each breast gets coated in the marinade. Let marinate at room temperature for 20 minutes.
4. Meanwhile, prepare the relish. In a mixing bowl, toss the red onions with the lemon juice. Let rest for 5 minutes just to mellow the onion’s “bite.” Add the mango, cherry tomatoes, cilantro, mint, and a pinch of salt. Gently toss to combine. Cover and refrigerate while you grill the chicken.
5. Preheat a grill (or grill pan) over medium-high heat. Remove the chicken from the marinade and place it on the hot grill. Cook for 3 to 4 minutes per side or until cooked through. (If using a grill pan, you will likely need to cook the chicken in batches.)
6. Serve the chicken warm or at room temperature topped with relish.