Pat the chuck roast dry with paper towels. Season generously on all sides with salt and pepper. This dry surface is essential for a proper sear.

Heat a heavy skillet over medium-high heat until it shimmers. Add the oil and swirl to coat. Sear the roast for 3 to 4 minutes per side, until a deep brown crust forms. Do not crowd the pan; work in batches if needed. Transfer to a plate.

In a small bowl, mix the chili powder, cumin, smoked paprika, garlic powder, onion powder, and cayenne if using. This is your spice blend. It's heavier on cumin than most store-bought mixes, which gives the beef a warm, earthy backbone.

Place the sliced onion and smashed garlic in the bottom of the slow cooker. Nestle the seared roast on top. Sprinkle the spice blend evenly over the meat.

In a separate small bowl, whisk the beef broth and tomato paste until smooth. Pour around the sides of the roast, not directly over the spices, so the seasoning stays on the meat. Add the bay leaves.

Cover and cook on LOW for 8 hours, or on HIGH for 5 hours. The meat should pull apart easily with two forks. Do not open the lid during the first 4 hours—each peek releases heat and extends cooking time.

Carefully transfer the roast to a cutting board. Let rest for 10 minutes. Meanwhile, strain the cooking liquid through a fine-mesh strainer into a fat separator or a bowl. Discard the solids. Skim off the fat, or use a large spoon to remove the top layer. Reserve 1 cup of the defatted liquid.

Using two forks, shred the beef into bite-sized pieces. Discard any large chunks of fat. Return the shredded meat to the slow cooker, add the reserved liquid, and toss to coat. Keep on the WARM setting until ready to serve.

Warm the tortillas directly over a gas flame for 30 seconds per side, or in a dry skillet until lightly charred. Fill each tortilla with a generous portion of beef and your chosen toppings.