Preheat oven to 300°. Pat beef dry with paper towels. Sprinkle beef with garlic salt and pepper; toss with flour and set aside.

Heat 1 tablespoon oil in a 5-quart Dutch oven over medium heat. Add mushrooms and onions; cook for 4 to 5 minutes or until softened, stirring often. Transfer to a large bowl.

Heat 1 tablespoon oil in Dutch oven over medium-high heat. Add half of beef and cook for 3 to 4 minutes or until brown, stirring occasionally. Transfer to a bowl with mushroom mixture. Add remaining 1 tablespoon oil and brown remaining beef; transfer to bowl.

Add 1 cup beef broth to Dutch oven. Cook over medium heat for 1 to 2 minutes, scraping and stirring brown bits from bottom of Dutch oven. Stir in remaining 2 cups beef broth, beer, and tomato paste.

Add beef, mushroom-onion mixture, potatoes, carrots, 2 tablespoons thyme, and bay leaves to broth mixture. Bring to a gentle simmer. Cover and transfer to oven. Bake for 1½ to 2 hours or until beef and vegetables are tender. Remove from oven.

Whisk together corn starch and vinegar; stir into stew. Simmer on stovetop for 1 to 2 minutes or until slightly thickened. Remove and discard bay leaves. Garnish with additional chopped thyme, if desired. Serve with biscuits, if desired.