Place bacon in a skillet and heat skillet over medium-high heat. Cook the bacon, adjusting heat as necessary until the bacon is is crisp.
Transfer bacon to a paper towel-lined plate. Reserve 1 tablespoon drippings and refresh with 1 tablespoon olive oil.
Generously season ribs with salt and black pepper. Continuing on medium-high heat, place half the ribs in the skillet. Brown well on both sides. Transfer to a slow cooker. Repeat with remaining ribs.
Add wine, broth, tomato paste, bay leaves, thyme, carrots, garlic and bacon to slow cooker.
Cook for 4-6 hours on HIGH heat or up to 8 hours on LOW heat.
Approximately half an hour before ribs are done, add thawed frozen pearl onions. Place flour in a small bowl. Remove 4-6 tablespoons of the hot liquid from the slow cooker. Stir 1 tablespoon of the hot liquid at a time into the flour until you have a thick paste.
Add the slurry (liquid and flour mixture) to the slow cooker and gently stir it in.
Cook uncovered on HIGH for 20 more minutes or until thickened and pearl onions are heated through.
Heat butter in a small skillet or saute pan. Add the mushrooms and cook over high heat or until tender and they release their juices. Add to slow cooker and gently stir.
Shut slow cooker off and let stand for 10 minutes or so. Taste and adjust for seasoning.
Serve over mashed potatoes, cauliflower puree or polenta.