Heat a large skillet over medium-high. Pat the beef dry, then cook until browned on all sides, 8 to 10 minutes. Transfer to a lined slow cooker.
Add the onions to the hot skillet and cook 2 minutes without disturbing. Add the butter and garlic. Season and cook, turning occasionally, until the onions are softened, 10 minutes. Stir in the stock and beer and simmer for 2 minutes; transfer the mixture to the slow cooker along with the potatoes and carrots. Cook on high for 4 hours or on low for 8 hours; season. Sprinkle with parsley before serving.