Heat the oil in a large skillet over medium-high heat. Place the roast into the hot oil and cook, turning frequently, until browned on all sides. Remove from the skillet and place in a slow cooker.
Cook garlic and onions in the skillet in the meat drippings until tender. Stir in the salt and tomato sauce. Combine the sugar, flour, cocoa powder, chili powder, oregano, cumin, coriander, cinnamon, and orange zest; stir into the tomato sauce. Pour the tomato sauce over the roast in the slow cooker. Add potatoes, carrots and celery. Cover, and cook on Low for 6 to 8 hours, or until meat is tender. Garnish with sliced almonds before serving.
