Place the chicken breasts on a cutting board and trim off excess fat. Cover the chicken with a large sheet of plastic wrap or wax paper. Using a meat mallet or the bottom of a heavy skillet, pound the chicken until it is a uniform 1/2-inch thick. This helps tenderize the chicken and makes it cook more evenly.
Transfer the chicken to a large bowl or zip-top bag. Add the oil, coconut aminos, lime juice, salt, chili powder, ground cumin, garlic powder, and black pepper and toss until well coated. Cover and refrigerate for at least 1 hour or up to 24 hours.
Place the tomatillos, halved onion, and jalapeño peppers on a small skillet and drizzle with oil.
Heat a grill over medium-high heat (350°F to 400°F). On one half of the grill, place the skillet with the tomatillos. Place the chicken on the other half of grill. Cover the grill and cook the chicken for about 4 minutes per side until the internal temperature reaches 165°F. Meanwhile, grill the veggies on the skillet until they begin to blister and are just tender, about 15 minutes total, rotating every 3 to 4 minutes. Remove from heat and set aside.
Remove the stem from the jalapeño peppers. In a food processor or high-powered blender, add the charred tomatillos, onions, jalapeño peppers, cilantro, lime juice, salt, and cumin. Blend until just smooth. Transfer to a bowl for serving.
Slice the chicken and serve in warmed tortillas with sliced avocado, diced onions, chopped cilantro, and tomatillo salsa. Serve with lime wedges and enjoy!