Combine chili powder, smoked paprika, oregano, cumin, cayenne, salt, and black pepper in a small bowl and set seasoning mix aside
Cut beef stew meat down into 1-inch bite sized cubes. Add two tablespoons oil to a large Dutch oven and place over MEDIUM-HIGH heat. Add half of the beef cubes to the hot oil and sprinkle with about 1/3 of the seasoning mix. Cook, stirring as needed, until browned on all sides, about 6 to 8 minutes. If the spices begin to burn, reduce the heat under the Dutch oven a bit and keep stirring. Transfer the beef to a plate and set aside. Repeat with remaining beef, adding additional oil, if needed, and another 1/3 of the seasoning mix (reserve remaining seasoning mix for later) .
Add more oil to the pan if necessary and add onion. Cook and stir until softened, 3 to 5 minutes. Add garlic and cook for just another minute or until fragrant. Add the tomato paste and cook another minute or two and then add the tomato sauce, beef broth, and remaining seasoning mix and stir until well combined. Let sauce simmer for two to three minutes. Return beef to the pan, bring sauce to a low boil, then reduce heat to low. Cover and simmer for 1 1/2 to 2 hours, or until beef is fork tender.
Meanwhile, place sweet potatoes into a large pot and cover with water; bring to a boil. Reduce heat to medium-low and simmer at a low boil until tender all the way through when pierced with a sharp knife, 40 to 45 minutes.
When cool enough to handle, transfer sweet potatoes to a cutting board and use paper towels to pull off the skins. Transfer the peeled potatoes to a large mixing bowl and use a hand mixer to combine them with the sour cream, butter, and salt, to taste.
Ladle the beef and sauce over mounds of sweet potatoes and garnish with green onion.