Place chicken in a shallow glass dish. Combine remaining ingredients and salt and pepper to taste in a small bowl. Pour marinade over chicken and turn pieces until thoroughly coated. Cover and let marinate in refrigerator for at least 2 hours, and up to 24 hours, turning occasionally.
Heat oven to 400°. Remove chicken from marinade and place in a single layer in a large baking dish, skin side up. Reserve remaining marinade for basting. Bake until skins are crispy, and chicken is cooked through (the thighs should have an internal temperature of 175°), 40–45 minutes, basting with reserved marinade halfway through cooking time. If the chicken isn't browning sufficiently, place under broiler for the last 5 minutes of cooking, or until crispy and browned.
Adjust the top oven rack to be closest to the heat source. Set oven to broil and heat for approximately 5 minutes. Place tomatillos (cut-side down), garlic cloves, and jalapeño (skin-side down), on a roasting pan. Broil until skins are beginning to char, 1–2 minutes on each side.
Transfer tomatillos and jalapeño into a blender or food processor. Squeeze roasted garlic out of their peels and add to blender. Let mixture cool to room temperature before adding cilantro, ¼ cup water, and lime juice. Blend to a coarse purée.
Pour salsa into a bowl and add onion. Salt to taste, and thin with additional water, as necessary, to reach your desired consistency.
Serve Tomatillo Salsa alongside Mexican Roasted Chicken, to be spooned on top.