Trim any excess fat from the roast and discard. Cut eight 1-inch slits into the roast (four on the top and four on the bottom). Tuck a garlic half into each slit.
In a large skillet, heat the olive oil over medium-high heat. Add the roast and cook 1 to 2 minutes on each side, achieving a deep brown color before turning to the next side.
Add the onion wedges to the slow cooker and place the browned roast on top. Pour the crushed tomatoes on top of the roast and sprinkle with the rosemary, salt and pepper. Cover and cook on high heat for 6 hours, or on low heat for 10 hours. Skim the top of the liquid in the slow cooker for any accumulated fat and discard. Use two forks to pull apart the meat. Serve topped with the sauce.