In 1-cup (250 mL) measuring cup, pour boiling water over dried porcini mushrooms; let stand for 20 minutes. Strain, reserving liquid. Set mushrooms and liquid aside.
Cut ribs into 2-bone pieces. Broil, turning once, until well browned, about 5 minutes. Place in slow cooker.
In large skillet, heat oil over medium-high heat; sauté porcini and button mushrooms, onions, garlic, carrots, rosemary, salt and pepper until softened, 5 minutes. Scrape into slow cooker. Add tomatoes.
Whisk together reserved soaking liquid, wine and tomato paste; pour into slow cooker. Cover and cook on low for 7 to 8 hours or until meat is tender. Skim off fat.
In small bowl, whisk together flour, vinegar and 1/3 cup (75 mL) water; whisk into slow cooker. Cover and cook on high for 15 minutes or until thickened. Stir in parsley.