Preheat oven to 300°.
To prepare ribs, heat a large Dutch oven coated with cooking spray over medium-high heat. Sprinkle ribs with 1/2 teaspoon salt and 1/2 teaspoon pepper. Add half of ribs to pan; cook 8 minutes or until browned, turning occasionally. Remove from pan. Repeat procedure with remaining ribs; remove from pan.
Add onion to pan; sauté 8 minutes or until lightly browned. Add garlic; sauté 1 minute. Return ribs to pan. Add broth, wine, vinegar, sugar, and tomato; bring to a simmer.
Cover and bake at 300° for 1 1/2 hours or until tender. Cool slightly. Refrigerate 8 hours or overnight.
Skim fat from surface of broth mixture; discard fat. Cook over medium heat 30 minutes or until thoroughly heated. Stir in 1/2 teaspoon salt and 1/2 teaspoon pepper.
To prepare mashed potatoes, place potatoes in a large saucepan; cover with water. Bring to a boil; cook 20 minutes or until very tender. Drain. Combine potatoes, milk, and remaining ingredients in a large bowl. Mash potato mixture with a potato masher. Serve with ribs and cooking liquid.