Season and flour beef:
Season the beef cubes with salt and pepper, then coat with 1/4 cup flour (30g).Brown beef in batches:
Heat oil in a skillet over medium heat. Working in batches, brown the meat until golden on all sides. Set aside.Deglaze skillet with broth:
Pour a small amount of broth into the skillet and scrape up any browned bits. Reserve this deglazed liquid.Sauté onion and garlic:
Melt butter in the same skillet. Sauté the diced onion and minced garlic until softened.Cook in slow cooker:
Transfer all ingredients to a slow cooker: browned beef, deglazed liquid, sautéed onion and garlic, quartered onion, whole garlic cloves, Worcestershire sauce, Guinness, bay leaves, rosemary, carrots, celery, and potatoes. Cover and cook on HIGH for 4 hours or LOW for 8 hours, until beef is fork-tender.Garnish with parsley:
Sprinkle with fresh parsley and serve hot.