In a large dutch oven, heat some olive oil over medium heat. Toss the meat cubes with the 1 ½ tbsp of flour, and then in batches, brown them until seared. Remove to plate as you go.
Add more oil if you need to, then add the celery, onion, and garlic. Cook and stir until the vegetables are soft. Add the thyme and season with salt and pepper. Stir in the mushrooms, bay leaves, broth, tomato paste, and Guinness; bring to a boil, then cover and put into the oven.
Let the stew bubble away in the oven for two hours.