Preheat the oven: Set to 375°F (190°C).
Lightly grease a 9x13-inch baking dish.
Sauté aromatics: Warm olive oil in a large skillet over medium heat. Add onion and cook until translucent, 3–4 minutes. Stir in garlic and cook 30 seconds until fragrant.
Cook the spinach: Add chopped spinach, a pinch of salt, and cook until wilted, 2–3 minutes.
If the pan looks watery, keep cooking another minute to evaporate excess moisture.
Season the filling: Stir in cumin and chili powder. Add shredded chicken and toss to warm through. Taste and season with salt and pepper.
If using, fold in Greek yogurt or sour cream for a creamier filling. Optional: add black beans or corn now.
Warm the tortillas: To prevent cracking, wrap tortillas in a damp paper towel and microwave 30–45 seconds, or briefly toast each side in a dry skillet.
Prepare the baking dish: Spread 1/2 to 3/4 cup enchilada sauce on the bottom of the dish to coat.
Assemble the enchiladas: Place a warm tortilla on a board, add a generous scoop of filling (about 1/3 cup), sprinkle a little cheese, and roll snugly. Place seam-side down in the dish.
Repeat with remaining tortillas and filling.
Sauce and cheese: Pour the remaining enchilada sauce evenly over the rolled tortillas. Top with the rest of the cheese, spreading it to the edges for even browning.
Bake: Cover loosely with foil and bake for 15 minutes. Remove foil and bake another 10–12 minutes, until the cheese is melted and bubbly and the edges are lightly browned.
Rest and serve: Let sit 5 minutes.
Garnish with chopped cilantro and serve with lime wedges. Add extra sour cream or hot sauce if you like.