Add the tomatoes, half of the onion, lime juice, achiote paste, 2 cloves of the garlic, cumin, salt and pepper to a large blender (or food processor) and blend until smooth.

Add the chicken to a large ziplock bag, then pour in the marinade and push as much air out of the bag as possible then seal. (You can also place the chicken in the marinade in a large bowl and cover with a lid or saran wrap.)

Let it marinate in the fridge for at least 3 hours, or overnight.

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

Heat the oil in a large skillet, over medium heat, then cook the onion until translucent, about 5 minutes. Stir in the garlic and cook for 30 seconds, or until it becomes fragrant.

Add the chicken with the marinade to the pan and let it sit without moving the chicken for about 5 minutes.

Flip the chicken over and let mixture simmer for another 8-10 minutes, or until the sauce has thickened to your liking.

Serve the chicken over arroz blanco and garnish with freshly minced cilantro, if desired.