If needed, pound thickest part of chicken breast to ½-inch first then season both sides with 1/2 tsp salt & 1/4 tsp pepper, or to your taste.
Heat the olive oil in a large skillet over medium-high heat (see Crockpot instructions in notes)
Once shiny, add the chicken and sear for 3 minutes undisturbed.
Flip the chicken. Add the onions and minced garlic. Saute, stirring occasionally for 3-4 minutes.
Stir in the crushed tomatoes, chopped chipotles, oregano, paprika, cumin and salt. Reduce heat to medium-low and bring sauce to a simmer for 10-15 minutes to thicken.
Remove chicken and shred with two forks, then toss back with the sauce until well coated.
Preheat the oven to 425 degrees F.
Wrap tortillas in a damp paper towel and warm in the microwave for 30-45 seconds.
Grease a baking sheet with olive oil spray. Lay the tortillas out. One one side of each tortilla spread about 1/3 cup(64g) chicken tinga. Sprinkle 2 tbsp(14g) shredded cheese on top.
Fold the other half of the tortilla over, seal gently with a fork then flip taco over. Repeat for all.
Bake for 5 minutes. Remove from the oven and flip tacos over. Bake for another 5 minutes, until the cheese is melted and the tacos are golden brown.
Serve with sour cream, taco sauce, shredded lettuce, etc and enjoy!