Make the sauce: Heat the oil in a medium saucepan over medium heat until it shimmers. Add the flour and stir continuously for 60 seconds until slightly golden and faintly nutty. Add the chili powder, cumin, garlic powder, oregano, and salt; stir 30 seconds until the spices bloom. Pour in the chicken broth slowly, whisking constantly to prevent lumps. Add the tomato sauce, bring to a gentle simmer, and cook 3 to 4 minutes until the sauce lightly coats the back of a spoon. Taste, adjust salt, and remove from heat.
Cook the chicken: Place the chicken thighs in a small saucepan and pour the 1 cup of chicken broth over them. Add the cumin, smoked paprika, garlic powder, and salt. Bring to a steady simmer over medium heat, cover, and cook 15 to 18 minutes, until fully cooked through (74°C / 165°F). Transfer to a cutting board and rest 3 minutes.
Shred and build the filling: Use two forks to shred the rested chicken along the muscle fibers. Transfer to a bowl and add the finely diced onion, 1/3 cup of the prepared enchilada sauce, and the 1/2 cup of shredded cheese. Toss until well coated and the filling holds together loosely.
Prepare the dish: Preheat the oven to 190°C (375°F). Spread 1/3 cup of the enchilada sauce in a thin, even layer across the bottom of a 9x13 inch (23x33cm) baking dish.
Fill and roll: Warm corn tortillas first (stack of four wrapped in a damp paper towel, microwave 30 to 45 seconds). Spoon about 3 tablespoons of filling down the center of each tortilla, leaving 2cm clear at each end. Roll snugly and place seam-side down in the dish, packed tightly side by side.
Sauce, cheese, and bake: Pour the remaining sauce evenly over the rolls, covering every visible piece of tortilla. Scatter the 1 cup of shredded cheese over the top. Bake uncovered on the center rack for 20 to 22 minutes, until the cheese is melted with golden caramelized patches and the sauce bubbles at the edges.
Rest and serve: Leave the dish to rest for 5 minutes so the sauce settles. Lift each enchilada cleanly with a wide spatula, sliding it under the full length of the roll. Serve with sour cream, guacamole, and lime wedges if you like.