Add meat, onion, garlic, carrot, rosemary and bay leaves to the bowl of your slow cooker
In large measuring cup, mix broth, salt, pepper, and Dijon mustard; pour over meat and veggies, then pour in red wine and red wine vinegar
Cover and place the bowl in the refrigerator overnight.
The next morning, set your stew to cook on low for 8 hours or on high for 6 hours.
1 to 2 hours before your stew is finished cooking, add the sliced mushrooms and pearl onions.
When your stew is done cooking, ladle some of the cooking liquid into small saucepan and bring to the boil.
Meanwhile, in a small container, mix tapioca starch and water together. Pour that into the boiling liquid and cook for a minute, whisking constantly, until thickened.
Transfer this thickened liquid back to the slow cooker and stir until well incorporated.
Serve immediately. (don't forget to fish out rosemary sprigs and bay leaves)