Pour water into a large saucepan and place over medium-high heat.
Add both tomatoes, 6 whole cloves of garlic, the chopped onion, salt and pepper to the water. Bring to a boil.
Add the chicken breasts to the water, and reduce heat to a gentle simmer. Poach until the chicken is mostly cooked through, about 20-30 minutes. (The chicken needs to be cooked enough to shred, but you don't want to overcook it.) Skim off any foam that may rise to the surface. Once the chicken is cooked, remove from the poaching liquid and set aside to cool.
Carefully pour the poaching liquid into a blender, being careful not to burn yourself. You can do this in batches if needed. Blend on high until the mixture is smooth, and set aside.
Add oil to a saucepan, and place over medium heat. Add the sliced onion and saute until soft and golden brown.
Add the 4 sliced cloves of garlic, and cook for 1-2 minutes, or until softened.
Add the blended poaching liquid, and bring to a hard simmer. Cook until the amount of liquid has reduced by half, and the mixture has thickened. about 10-15 minutes.
Meanwhile, remove and discard the skin and bones from the chicken breasts. Shred the meat with your fingers and set aside.
Once the sauce has thickened, add the minced chipotle pepper and adobo sauce, and stir to combine. Then add the shredded chicken and cook until the chicken is coated and heated through, being careful not to overcook the meat. Adjust the seasoning, if needed.
Slice a baguette or submarine roll into thirds. Brush with melted butter and toast in a toaster oven, or add cold butter to a pan over medium high heat, allow the butter to melt, then toast the bread in the pan.
Cut open toasted bread, spread with mayonnaise and spoon the chicken tinga mixture inside. Serve immediately. Refrigerate leftover portions of meat promptly.
Chicken Tinga can also be enjoyed in tacos, burritos, nachos, or just served simply with refried beans on a tortilla.Note: Chicken is fully cooked and safe to eat at 165° Fahrenheit (75° Celsius). Please note that the nutritional information for this recipe is only an estimate, and has been calculated using a plugin.