Step 1

Preheat oven to 400°F. Combine broth, half of the onion, and 4 of the oregano sprigs in a medium saucepan over medium-high; bring to a boil.

Step 2

Add chicken to pan; reduce heat to medium-low, and keep cooking liquid between 160°F and 180°F. Cook chicken until a thermometer inserted in thickest portion registers 160°F, 25 to 30 minutes. (Keeping the cooking liquid at the right temperature is important. You want it to be steamy, but not bubble at all.)

Step 3

Remove chicken from cooking liquid, and let cool 15 minutes. Shred chicken. Reserve 2 cups of the cooking liquid.

Step 4

Finely chop remaining half onion (about 3/4 cup). Combine tomato sauce, chile powder, garlic, flour, cumin, salt, chopped onion, and reserved 2 cups cooking liquid in a medium saucepan over high; bring to a boil. Cook, stirring often, until thickened, 1 to 2 minutes. Reserve 2 cups of the tomato mixture. Add shredded chicken and beans to remaining tomato mixture in pan; toss to coat.

Step 5

Spoon about 1/3 cup chicken mixture in center of each tortilla, and roll up. Arrange rolled tortillas, seam side down, in a 13- x 9-inch glass or ceramic baking dish coated with cooking spray. Evenly top with reserved 2 cups tomato mixture; sprinkle with cheese. Bake in preheated oven until cheese is melted and sauce is bubbly, about 20 minutes. Top with chopped tomatoes, avocado, and leaves from remaining 2 oregano sprigs (about 2 Tbsp.). Dollop with sour cream.