Heat the olive oil in a large soup pot or dutch oven.
Saute the onion and bell pepper with about 1/2 tsp salt until they start to soften, about 5 minutes.
Stir ancho chili powder and tomato paste and stir until everything is coated in the tomato paste and spice.
Sprinkle masa over the veggie mixture and stir together. Cook for 1-2 minutes so that the flour is no longer raw. It should be a thick glob of a roux at this point.
Slowly begin adding the broth and whisking it together with the roux, making sure to avoid any lumps. Once it begins to look smooth and saucy, pour the rest of the broth in and stir together.
Add the diced tomatoes.
Taste the soup BEFORE adding the raw chicken. Add salt to taste.
Add the chicken (it will cook in the broth).
Bring to a boil, then cover and reduce to a simmer.
Cook on low for 20-30 minutes, or until the chicken is cooked through and easily pulls apart.
Remove the chicken, shred it, and throw it back into the pot.
Let the soup cook for 5 more minutes, then serve with your favorite toppings!