Heat the vegetable oil in a large skillet over medium-high heat.
Add the beef cubes in batches and brown on all sides, about 4-5 minutes per batch. Transfer browned beef to the slow cooker.
In the same skillet, add the chopped onion and cook until softened, about 3 minutes.
Add the minced garlic and cook for 1 minute until fragrant.
Stir in the sliced carrots, celery, and mushrooms and cook for 5 minutes until vegetables begin to soften.
Sprinkle the flour over the vegetables and stir well to coat, cooking for 2 minutes to remove raw flour taste.
Slowly pour in the ale while stirring to deglaze the skillet, scraping up any browned bits from the bottom.
Add the beef broth and tomato paste, stirring to combine.
Pour the vegetable and ale mixture into the slow cooker over the beef.
Add the dried thyme, bay leaves, salt, and black pepper to the slow cooker and stir gently.
Add the potato chunks to the slow cooker and mix gently to combine.
Cover and cook on low for 8 hours, or until the beef is tender and the vegetables are cooked through.
Remove the bay leaves before serving.
Sprinkle chopped fresh parsley over the stew and serve hot.