Pat the beef oxtails dry with paper towels. Trim away any hard excess fat, leaving some of the softer fat in place. Season generously with your favorite beef rub.
Preheat the smoker to 275°F and add apple wood chunks.
Place the oxtails on the smoker and smoke for about 3 hours, or until a deep, dark bark develops.
About 2 hours into smoking, heat a Dutch oven over medium-low heat. Cut the tomatoes in half and the onion into quarters. Add them to the Dutch oven with a little olive oil and cook for about 15 minutes.
Add the garlic, red wine, Worcestershire sauce, thyme, oregano, beef broth, salt, and pepper. Simmer for about 20 minutes.
Add the guajillo and ancho chiles and simmer for another 15 minutes.
Transfer the solids from the Dutch oven to a food processor along with about 1 cup of the braising liquid. Blend for 60–90 seconds.
Pour the blended mixture back into the Dutch oven. Add the smoked oxtails, cinnamon stick, bay leaves, and allspice berries.
Cover and braise at about 275°F for 4–5 hours, or until the oxtails are very tender and easily pull apart.
Cut the potatoes into quarters. Bring 4–6 cups of salted water to a boil and cook the potatoes for 12–15 minutes, or until fork-tender.
Drain the potatoes and return them to the pot. Add the butter, sour cream, shredded cheese, chives, salt, garlic powder, and black pepper. Mash until creamy.
Remove the oxtails from the Dutch oven and place them on a wire rack. Strain the braising liquid, separating the solids from the liquid.
Spoon the creamy mashed potatoes onto plates and top with the oxtails. Pour some of the strained braising liquid over the beef and garnish with fresh parsley.