Rub brisket with salt and pepper. Heat a large heavy skillet over medium-high heat. Coat pan with cooking spray. Add brisket to pan; cook 10 minutes, browning on all sides. Remove brisket from the pan. Add 1/4 cup water to pan, stirring to loosen browned bits. Add onion and parsnip; sauté 5 minutes or until vegetables are tender.
Place onion mixture, vinegar, bay leaf, and beer in a large electric slow cooker. Place brisket on top of onion mixture. Cover and cook on low for 8 hours. Discard bay leaf. Cut brisket diagonally across the grain into thin slices. Serve with sauce.