In a large heavy-bottomed pot or Dutch oven, melt the butter over medium-high heat. Add the beef cubes and sear until browned on all sides. Do this in batches if necessary to avoid overcrowding.

Remove the beef from the pot and set aside. Add the chopped onion to the remaining fat in the pot and sauté for 3-4 minutes until softened.

Pour in the stout beer to deglaze the pot, scraping up any browned bits from the bottom with a wooden spoon.

Return the beef to the pot and add the beef broth, dried thyme, and bay leaf. Bring to a boil, then reduce heat to low, cover, and simmer for 60 minutes.

Add the diced potatoes, carrots, and rutabaga to the pot. Stir well to combine.

Continue to simmer, covered, for another 45 to 60 minutes, or until the vegetables are very tender and the stew has thickened naturally from the starch of the potatoes.

Remove the bay leaf. Season generously with salt and black pepper to taste.

Serve hot in bowls, garnished with fresh parsley.