In a large, heavy-bottomed pot or Dutch oven, melt 6 tablespoons (90 ml / 3 fl oz) of the butter over medium heat.
Add the sliced leeks, whole boiling onions, and sliced parsley root.
Sauté gently, stirring occasionally.
Once the vegetables begin to soften, stir in the minced garlic, thyme, bay leaves, and rosemary.
Cook for another 2-3 minutes until fragrant and the leeks are turning a lovely golden color.
Add the mushrooms and diced turnips to the pot, stirring to coat.
Pour in the white wine and vegetarian Worcestershire sauce.
Bring to a simmer, then reduce the heat to low.
Gently stir in the diced potatoes and trimmed brussels sprouts.
Place the lid on the pot and let the vegetables begin to soften while you prepare the sauce.
In a separate small saucepan, melt the remaining 3 tablespoons (45 ml / 1.5 fl oz) of butter over medium heat.
Whisk in the flour to form a paste (a roux) and cook for 2 minutes, stirring constantly. Gradually pour in the hot vegetable broth while whisking vigorously to prevent lumps.
Once smooth, whisk in the wine vinegar, molasses, paprika, and a dash of hot sauce.
Pour the finished sauce over the vegetables in the large pot.
Stir everything together gently to combine.
Cover the pot and let the stew simmer gently for about 1 hour, or until all the vegetables are fork-tender.
Stir occasionally to prevent sticking.
Remove the bay leaves before serving.
Season generously with salt and pepper to your liking.
Serve piping hot, preferably with a side of crusty bread for dipping.