Heat a large skillet over medium high heat. Once the pan is hot, add 1 tablespoon of the oil. Sprinkle beef with pepper and 1 teaspoon salt. Add half of beef to skillet, and cook, until browned on both sides, 3 to 4 minutes per side. Transfer beef to the slow cooker. Repeat with remaining 1 tablespoon oil and remaining half of beef.
Add mushrooms to the skillet, and cook, stirring often, until browned, about 5 minutes. Transfer the mushrooms to a plate, cover and refrigerate.
Add onions and carrots to the skillet. Cook, stirring occasionally until onion begins to brown, about 5 minutes. Add smashed garlic and cook until fragrant, about 1 minute.
Add the red wine and cook until the liquid has almost evaporated, about 4 minutes. Stir and scrape to loosen browned bits from bottom of skillet.
Add red wine vegetable mixture to slow cooker with beef. Stir in coriander seed bundle, bay leaves, potatoes, tomato paste and beef stock. Cover and cook on LOW for 7 hours.
Once the stew has cooked for 7 hours, fish out 1 cup of beef stock and place it in a bowl. Whisk in the flour to form a thick paste roux. Add the roux back into the stew and use the whisk to blend it in. Add the fried mushrooms, whole grain mustard, and vinegar. Turn up heat to high and cook for an additional 15 minutes.
Remove the coriander seed bundle and throw away. Ladle into bowls and garnish with chopped chives or chopped flat leaf parsley.