Cut the beef steak into 2-inch cubes and season generously with salt and black pepper. Heat 2 tablespoons butter in a large Dutch oven over medium-high heat. Brown the beef in batches, ensuring not to overcrowd the pan, about 4 minutes per side. Remove and set aside. In the same pot, add remaining butter and sauté the diced onions until golden and caramelized, about 8-10 minutes. Add minced garlic and cook for 1 minute until fragrant. Sprinkle flour over the onions and stir to create a roux, cooking for 2 minutes. Slowly pour in the dark beer while stirring continuously to prevent lumps. Add bay leaves, thyme, and a pinch of nutmeg. Return the browned beef to the pot along with any accumulated juices. Bring to a simmer, then reduce heat to low, cover, and braise for 2 hours, stirring occasionally. Meanwhile, cut chicory lengthwise into quarters and remove the bitter core. In a separate skillet, melt 1 tablespoon butter and sauté the chicory until lightly caramelized, about 6-8 minutes. Add mushrooms and cook until golden, another 5 minutes. After 2 hours, add the chicory and mushrooms to the stew. Continue cooking uncovered for 30 minutes until the sauce has thickened and the beef is fork-tender. Adjust seasoning with salt and pepper. Prepare mashed potatoes by boiling potatoes until tender, then mashing with butter and milk until creamy. Serve the stoofvlees over mashed potatoes, garnished with fresh parsley.