In a large skillet, heat the olive oil over medium-high heat. Add the beef cubes and brown them on all sides. This should take about 5-7 minutes. Transfer the browned beef to a slow cooker.
In the same skillet, add the chopped onion and garlic. Sauté for about 3-4 minutes until the onion is translucent. Add this mixture to the slow cooker with the beef.
Add the carrots, potatoes, and mushrooms to the slow cooker. Pour in the beef broth and Belgian ale. Stir in the tomato paste, brown sugar, thyme, rosemary, bay leaf, salt, and pepper.
Cover the slow cooker and cook on low for 7-8 hours or on high for 4-5 hours, until the beef is tender and the flavors have melded together.
Before serving, remove the bay leaf and taste the stew. Adjust seasoning if necessary. Garnish with fresh parsley before serving.