Bring a large pot of water to a boil. Salt the water and drop in 2 large chicken breasts. Allow the chicken breasts to boil for about 30 minutes until they are fully cooked.
While the chicken boils, make the spicy tinga sauce. Add 1 cup canned crushed tomatoes, 2 chipotle peppers in adobo sauce, and 2 garlic cloves (peeled) to a blender. Blend on the medium setting until smooth. To turn up the spice level, you can add extra adobo sauce from the can of chipotle peppers.
Thinly slice 1/2 white onion. Then, heat 1/4 cup avocado oil (or similar oil with a high smoke point like vegetable oil) in a large pan over medium-high heat. Add the sliced onion and cook it for 6-8 minutes until soft and translucent.
Add the spicy tinga sauce from the blender to the pan. Fold the onions into the sauce and reduce heat to low allowing it to simmer until the chicken is ready.
When the chicken breasts are fully cooked, transfer them to a plate or cutting board and shred them using two forks. It is easier to shred when the chicken is hot out of the pot, so shred it immediately.
Fold the shredded chicken into the pan with the spicy chipotle sauce and onions.
Add salt to taste and serve with warm tortillas or with rice.