Layer the base: In the slow cooker, add the sliced onion, garlic, cumin, chili powder, salt, pepper, salsa verde, and chicken broth. Stir to combine.
Add the chicken: Nestle the chicken breasts into the sauce mixture, turning once to coat.
Cook low and slow: Cover and cook on Low for 4–6 hours or on High for 2.5–3.5 hours, until the chicken is very tender and easy to shred.
Shred the chicken: Transfer the chicken to a plate and shred with two forks.
Return the chicken to the slow cooker and stir to coat in the sauce.
Make it creamy (optional): Stir in the sour cream or Greek yogurt for a richer, creamier filling. Taste and adjust salt if needed.
Warm the tortillas: Wrap tortillas in a damp paper towel and microwave 30–45 seconds, or warm on a skillet. This prevents cracking when rolling.
Assemble the enchiladas: Spread a thin layer of the verde sauce from the slow cooker on the bottom of a 9x13-inch baking dish.
Place a spoonful of chicken and a sprinkle of cheese into each tortilla, roll snugly, and line them up seam-side down in the dish.
Sauce and cheese: Spoon more verde sauce over the enchiladas and top with the remaining cheese.
Bake: Bake at 375°F (190°C) for 15–20 minutes, until the cheese is melted and bubbly. If you prefer to keep it one-pot, place rolled tortillas back into the slow cooker, top with sauce and cheese, cover, and cook on High for 15–20 minutes until melted.
Finish and serve: Sprinkle with chopped cilantro, add a squeeze of lime, and garnish with avocado, jalapeños, or extra salsa. Serve hot.