Rub brisket with salt and pepper.
Coat a large, heavy skillet with a little olive oil over medium-high heat; add brisket and cook for 10 minutes, browning on all sides; remove from pan.
Add the water to the skillet and whisk up all of the browned bits from the bottom of the pan. Add onion and parsnips and sauté for 5 minutes or until tender.
In a slow cooker, place onion/parsnip mixture, vinegar, bay leaves and beer (or broth); place brisket on top. Cover and cook on LOW for 8 hours. Discard bay leaves.
Thinly slice brisket across the grain (diagonally); serve with sauce.