Combine first six ingredients together in a bowl. Rub spices all over beef chuck roast.
Heat olive oil in a skillet over medium heat. Sear seasoned chuck roast on all sides, just until browned.
Place sliced onion on the bottom of the slow cooker. Top with seared chuck roast. Pour beef broth and diced tomatoes over the top. Cook in the slow cooker on low for 8 hours.
After 8 hours use a slotted spoon to remove the tomatoes and onions. Reserve the tomatoes, onions and 1 1/2 cups of the pan juices. Add to a pot and cook over medium-high heat until reduced by 1/2, about 15 minutes.
Transfer pot roast to a serving platter, spoon tomatoes over the top. Serve.