Add the beef stew ingredients in the slow cooker - beef, carrots, celery, tomatoes, potatoes, red wine, salt, thyme, mustard powder, salt and pepper. Stir to combine.
Whisk together the flour and water until well blended, and stir into the stew.
Cover and cook on LOW for 8 to 10 hours (or High for 4-5 hrs).
When the stew is done, it's time to add the dumplings. In a medium bowl, mix baking mix, thyme and sage. Stir in milk just until it is just combined.
Drop dough in small spoonfuls (use a small cookie scoop) on top of the stew.
Switch the heat setting to high, and cover.
Cook until a toothpick inserted in the dumplings comes out clean (about 20 minutes).