Season beef with salt and pepper. In Dutch oven, heat half of the oil over medium-high heat, brown beef all over, adding more of the remaining oil, if necessary. Transfer to slow cooker.
Drain fat from Dutch oven, add remaining oil. Fry onion, garlic, proscuitto and marjoram over medium heat until onion is softened, about 5 minutes. Add broth and sherry, bring to boil, scraping up brown bits from pan. Add to slow cooker along with tomatoes and red peppers. Stir to combine.
Cover and cook on low until beef is tender, 6 to 8 hours. Transfer beef to cutting board, cover and keep warm before slicing across the grain.
Meanwhile skim fat from liquid in slow cooker. In small bowl whisk flour with 1/4 cup (50 mL) water; whisk into liquid along with almonds and tomato paste. Stir in green pepper. Cover and cook on high until thickened, about 15 minutes. Serve with beef.