Begin by combining the Roma tomatoes, half of the onion, lime juice, achiote paste, two garlic cloves, cumin, salt, and pepper in a blender or food processor. Blend the ingredients until smooth, creating a well-balanced marinade.

Transfer the chicken thighs into a large ziplock bag, then pour in the prepared marinade. Seal the bag, removing as much air as possible, and refrigerate for at least 3 hours, or overnight for deeper flavor absorption. Alternatively, marinate the chicken in a covered bowl.

About 10 minutes before cooking, finely dice the remaining half of the onion and mince the last garlic clove.

Heat olive oil in a large skillet over medium heat. Add the diced onion and sauté for approximately 5 minutes, or until softened and translucent. Stir in the minced garlic and cook for an additional 30 seconds, allowing the garlic to become fragrant.

Add the marinated chicken thighs and all the marinade to the skillet. Let the chicken sit undisturbed for about 5 minutes to achieve a nice sear.

Flip the chicken over and continue to simmer for another 8-10 minutes, or until the sauce has thickened to your desired consistency.

Serve the chicken over a bed of arroz blanco, garnished with freshly minced cilantro for a pop of color and flavor.