Toss the beef cubes with the flour, salt, and pepper in a bowl until evenly coated.
Heat a large skillet over medium-high heat and brown the beef in batches, then transfer it to the slow cooker.
Add the sliced onions to the same skillet and cook until they soften and start to brown at the edges.
Pour the dark beer into the skillet and scrape up any browned bits, then transfer everything to the slow cooker.
Add the turnip cubes, carrot slices, beef broth, minced garlic, thyme, and bay leaves to the slow cooker.
Stir gently to combine, cover, and cook on low for 8 hours until the beef and turnips are tender.
Remove the bay leaves, taste, and adjust salt and pepper before serving.