Pat the beef cubes dry with paper towels and season with salt and pepper.
Heat the vegetable oil in a large skillet over medium-high heat. Add the beef in batches and brown on all sides, about 3-4 minutes per batch. Transfer browned beef to the slow cooker.
Add the sliced onions to the skillet and cook over medium heat, stirring occasionally, until caramelized and golden brown, about 15 minutes.
Add the minced garlic and cook for 1 minute until fragrant.
Sprinkle the flour over the onions and garlic, stirring constantly for 2 minutes to cook the flour.
Stir in the brown sugar and tomato paste, cooking for 1 minute more.
Pour in the dark beer, scraping the bottom of the skillet to loosen browned bits. Bring to a simmer and cook for 3-4 minutes until slightly thickened.
Transfer the onion and beer mixture to the slow cooker with the beef.
Add the beef broth, apple cider vinegar, bay leaves, and thyme sprigs to the slow cooker. Stir gently to combine.
Cover and cook on low for 7 to 8 hours, or until the beef is very tender.
Remove the bay leaves and thyme sprigs. Stir in the unsalted butter until melted and incorporated to enrich the sauce.
Taste and adjust seasoning with additional salt and pepper if needed.
Serve hot, garnished with chopped fresh parsley.