Melt the butter in a large saucepan over medium heat. Add the mushrooms and saute for 5 minutes, or until they are dark brown and tender. Add the beer, 1/2 cup water, bay leaf, thyme, ground black pepper and salt.
Add the meat, onions, green bell pepper and red bell pepper. Simmer for another 20 minutes, or until beef is tender. Remove bay leaf.
In a separate small bowl, combine the flour and remaining 1/4 cup water. Mix well, making sure the flour is completely dissolved. Stir this mixture into the meat mixture. Cook until thickened and bubbly, then cook for 3 minutes more.