Prepare the seasoning: In a small bowl, combine the white pepper, seven spices (or allspice), paprika, garlic powder, onion powder, black pepper, and salt.
Season the roast: Place the roast on a large tray. Drizzle 3 tablespoons of olive oil over the roast and rub it evenly into the meat. Use half of the spice mixture to generously season the entire roast, making sure to rub the seasoning into all the sides and creases.
Sear the roast: Heat a large skillet over medium-high heat and add the remaining 3 tablespoons of olive oil. Once hot, sear the roast for about 2 minutes per side, until nicely browned. This adds a deeper flavor to the finished roast.
Add the roast to the slow cooker: Sprinkle the remaining spice mixture into the bottom of the slow cooker, then place the seared roast on top. Add the bouillon cubes around the roast, if using.
Sauté the aromatics and vegetables: Using the same skillet and the flavorful drippings left behind, add the cloves, star anise, cinnamon sticks, onions, and carrots. Sauté over high heat for about 5–6 minutes, stirring occasionally, until the onions begin to brown and the spices become fragrant.
Assemble: Transfer the sautéed onions, carrots, and spices to the slow cooker, placing them around and over the roast. Add the cubed potatoes and tomatoes.
Slow cook: Cover and cook on HIGH for about 6 hours, or until the roast is fork-tender and the vegetables are cooked through. Cooking time can vary depending on the size and cut of your roast.
Serve: Carefully remove the roast, let it rest for a few minutes, then slice or shred and serve with the tender vegetables and flavorful juices from the slow cooker.