Preheat oven to 300°F. Pat chuck roast dry and season generously with salt, pepper, taco seasoning, cumin and paprika. Heat a cast-iron skillet over medium-high heat and sear roast for 3–4 minutes per side until well-browned. Transfer to a plate.
In the same skillet, add the onion and garlic and cook 1–2 minutes until fragrant. Stir in tomato sauce and beef broth, scraping any browned bits from the bottom of the pan. Taste and adjust seasoning as needed.
Return beef to the skillet, nestling it into the sauce. Cover with a lid or foil. Transfer to the oven and cook for 2–3 hours, or until the beef is very tender and shreds easily. Shred beef directly into the sauce and keep warm.
Increase oven temperature to 425°F. Toss sweet potatoes with olive oil, salt and seasoning. Spread in a single layer on a sheet pan and roast for 15–20 minutes, flipping halfway, until caramelized and golden.
Whisk together sour cream and Sriracha. Add a squeeze of lime for brightness, if desired.
Warm tortillas. Top with shredded beef and roasted sweet potatoes. Drizzle with spicy crema and finish with cilantro and a squeeze of lime. Serve with avocado if using.
