Toss the beef chunks with the flour, salt, and pepper until evenly coated. Heat a skillet over medium-high heat and brown the beef in batches, then transfer it to the slow cooker.
Add the onion, carrots, and garlic to the slow cooker along with the Guinness, beef broth, bay leaves, and thyme. Stir once to combine.
Cover and cook on low for 7 hours.
While the stew cooks, preheat the oven to 425°F. Toss the parsnip pieces with olive oil, spread them on a baking sheet, and roast for 25 minutes until the edges turn golden.
When the stew is done, remove the bay leaves. Stir in the roasted parsnips and the chopped sage. Let everything sit for 5 minutes before serving so the flavors settle.