Heat vegetable oil in a large skillet over medium-high heat.
Add beef cubes in batches and brown on all sides, about 3-4 minutes per batch. Transfer browned beef to the slow cooker.
In the same skillet, add sliced onions and cook over medium heat until caramelized, about 15 minutes, stirring frequently.
Add minced garlic and brown sugar to the onions and cook for an additional 2 minutes, stirring constantly.
Sprinkle flour over the onions and stir well to combine, cooking for 1 minute to remove raw flour taste.
Slowly pour in the dark beer, scraping the bottom of the skillet to loosen browned bits. Stir in beef broth, apple cider vinegar, and Dijon mustard.
Bring the mixture to a simmer, then transfer it to the slow cooker over the beef.
Add bay leaves, fresh thyme sprigs, salt, and black pepper to the slow cooker. Stir gently to combine.
Cover and cook on low for 8 hours, or until the beef is very tender.
About 30 minutes before serving, melt butter in a skillet over medium heat.
Toast the bread slices in the butter until golden brown on both sides.
Place one toasted bread slice on top of the stew in the slow cooker, pressing it down slightly to soak up the sauce. Cover and continue cooking for the remaining 30 minutes.
Remove bay leaves and thyme sprigs before serving.
Ladle stew into bowls, garnish with chopped fresh parsley, and serve hot.