In a slow cooker (crockpot), add beef roast, onion, poblano, chili powder, cumin, garlic, and beef broth. Cook on a low setting for 8 hours. After it is done cooking, shred with two forks.
Enchilada Sauce:
In a medium pot, saute onion in oil over medium heat until onion is soft, about 7-8 minutes. Stir in tomato sauce, chicken broth, chili powder, cumin, Mexican oregano, and garlic powder. Cook for 20 minutes until it starts to thicken and reduce. Carefully puree in a blender or with an immersion blender.
Preheat oven to 375 degrees. Spread a layer of enchilada sauce on the bottom of a 9 x 13 pan. Layer corn tortillas, shredded beef, enchilada sauce, and cheese. Repeat to make at least 3 layers. Top with a generous amount of cheese.
Cover with foil and bake for 20-25 minutes. Remove the foil and bake for 5-10 minutes until the cheese is bubbling.