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Rinse the leek, remove the outer leaves and a part of the greens leaves. Coarsely chop the leek, peel the carrots and also coarsely chop them.
Peel and finely chop the onion and garlic cloves. Remove the hard parts of the fennel bulbs and chop them into coarse pieces. Rinse the potatoes, there's no need to peel them.
Divide the meat into bite-size pieces and coat them with flour, pepper and salt.
Heat a knob of butter in a skillet and cook the stewing meat in batches until they're beautiful brown. Add the meat to the slow cooker along with onion, garlic, carrots, fennel, potatoes, leek and bay leaves.
Deglaze the skillet using red wine, this way all the meat flavours will end up in the slow cooker. Stir the mustard into the red wine along with the beef stock. Pour the mixture into the slow cooker and set to the lowest setting for about 5 hours.
You can also create this stew in a Dutch oven, it's best to sauté everything before you'll add the liquids and let it simmer for about 1,5 hours.
Season the stew with pepper and salt and remove the bay leaves before serving. Enjoy!