In a sauce pan over medium-high heat add oil and brown your beef stew in batches. Transfer to slow cooker.
Add bacon to same sauce pan and cook until crisp. Remove and drain on paper towel. Set aside. Once cool cut into 1 inch pieces and add to the slow cooker.
In pan with bacon fat add in your sliced onions. Cook until soft. Add garlic, thyme, old bay seasoning, bay leaves, vinegar, another dash of salt & pepper and cook for 1 minute or so. Sprinkle sugar over mixture in pan and stir well. Add flour and cook for another minute (stir as you go). Finally add your beer, increase your heat a little and cook until mixture thickens.
Pour saucepan mixture over beef & bacon. Gently stir and cook on low for about 5-6 hours or until meat is tender. Remove bay leaves and serve topped with fresh parsley.