Preheat oven to 325º. Put tomato paste into a large bowl, add stock, and stir until paste dissolves. Add beef, mushrooms, onions, garlic, thyme, and stout and stir well. Sprinkle flour over beef mixture, season to taste with salt and pepper, and stir well. Transfer to a deep 10 1/2" x 12 1/2" baking dish set on a large baking sheet. Cover dish with foil. Bake sirring occasionally, until meat is very tender, 3 1/2 - 4 hours.