To make the sauce, Put the tomatillos, chilies, and onion in a saucepan with the vinegar and water to cover. Bring to a boil, reduce the heat and poach until the tomatillos are soft, about 10 minutes. Drain. Put the vegetables in a blender, add the cumin, sugar and salt and puree. Add the cilantro, lime juice and pulse to combine. Pour sauce into a large skillet cook it over medium heat until thickened, about 5 minutes, stirring frequently to prevent sticking. Add the broth and simmer covered for about 10 minutes. Add sour cream and continue cooking uncovered until the sauce is slightly thickened and not watery, about 5 to 8 minutes. Add more salt if desired. Stir in cheese until just melted.
For the enchiladas: Heat a 2-count of olive oil in a medium saucepan over medium heat. Add the onion and cook until softened and caramelized, 5 to 7 minutes. Add the garlic and cook 1 minute. Stir in the cumin and cook 1 minute. Then sprinkle on the flour and cook, stirring, 1 more minute. Gradually pour in the stock, stirring constantly. Bring to a simmer, stirring to make sure the flour doesn't stick to the bottom of the pan; the liquid will thicken. Fold in the chicken, diced peppers, and cilantro, and season well with salt and pepper.
To assemble the dish: Preheat the oven to 350 degrees F. Get yourself a large baking dish. Dip a tortilla into the tomatilla salsa and put it on a cutting board. Put a big scoop of the chicken mixture in the center, sprinkle with a little of the cheese, and roll the tortilla like a cigar to enclose the filling. Use a spatula to place it seam side down in the baking dish. Continue to fill all of the tortillas and put them in the baking dish. Pour the remaining tomatilla salsa over the top and sprinkle with the remaining cheese. Bake uncovered for about 30 minutes until bubbly and cracked on top.