Combine garlic, lemon, lime, monk fruit, cumin, oregano, pepper, allspice, and salt in a large bowl and set aside.
Pound the chicken to and even 1/4 inch thickness between two pieces of plastic wrap. I often use a rolling pin or large bottle in this task. Add the chicken to the marinade and coat evenly, pressing to ensure it is all covered. Refrigerate for 30 minutes.
Heat a large pan over medium-high heat and when hot, add the oil. When the oil is shimmering, remove the chicken from the marinade, removing excess marinade and lay in the pan. Keep the chicken in a single layer and cook for 2-3 minutes per side. Work in batches if it won’t all fit in one round of cooking. Place on a serving plate. When all chicken is cooked, add the onions and a pinch of salt. Sauté until golden and soft, about 10 minutes. Add water if they are browning too fast. Serve on top of chicken with some cilantro for garnish.