Add chicken breasts to a gallon size zip top bag. In a medium bowl, combine marinade ingredients with a whisk. Pour into the bag with the chicken and seal, removing as much air from the bag as possible. Gently massage the chicken with your hands and flip bag back and forth to fully coat the chicken. Place the bag inside a bowl; refrigerate for at least 2 hours or overnight. Every so often, flip the bag over a few times to keep the chicken coated and moist, while marinating.
Slice onions and place them in the center of large sheet heavy-duty foil. Add butter, sprinkle with salt and pepper and then fold to make a packet.
Place chicken on the counter to come to room temp while the grill is heating. Heat grill with coals and brush the rack with oil to prevent the chicken from sticking. Remove meat from marinade and place the chicken on the grill; discard the leftover marinade. Cook the chicken 8 to 10 minutes on the first side. Turn and cook an additional 8 to 10 minutes. Turn chicken, as needed to keep from burning, until chicken is cooked through and tender, about 20 minutes.
Place onion packet on the grill. Cook entire time chicken is on the grill.
Brush poblano peppers with oil and place on the grill with the chicken. Roast the poblanos over the flame, turning them every 2 to 3 minutes for a total of 6 to 9 minutes. Remove the poblanos from the grill; place them into a bowl. Seal the bowl with plastic wrap to sweat.
Remove the chicken and onion packet from the grill. Let the chicken stand for 5 minutes before slicing.
While chicken is resting, remove the skins from the peppers remove seeds and slice into ¼-inch strips. Mix the peppers and onions from the foil packet in a bowl.
Slice the chicken against the grain into strips. Serve on a warm toasted tortilla, topped with the peppers and onions, and any toppings you choose.
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