In a large saucepan, heat the butter or oil over medium-high heat. Season the lamb with salt and pepper, and brown it on all sides. Pour off excess oil from the pan and add the Guinness, beef stock, garlic, bay leaf, and thyme. Using a wooden spoon, scrape the bottom of the pan, and bring the liquid to a boil. Reduce the heat, cover, and simmer for 20 minutes.
Stir in the squash, potato, celery, and onion. Bring the liquid back to a boil and then return to a simmer, cover, and cook for 30 minutes more, or until the lamb and vegetables are tender. Discard the bay leaf and thyme sprigs.
In a small combine the yogurt and flour. Stir 1/2 cup of the hot liquid into the yogurt mixture, and add the yogurt mixture to the saucepan. Cook and stir until the liquid is thickened, about 3 minutes. Season to taste with salt and pepper, and serve with crusty bread for dipping.