Dust the lamb in flour that has been seasoned witha teaspoon of salt and 1/4 teaspoon pepper. Reserve the flour that is left after dusting to use as a thicker at the end of the cooking process.
Heat a large heavy Dutch oven or similar vessel over medium high heat and add the butter or oil.
Sear the lamb until brown on all sides.
Add the veggies except the potatoes and continue to cook at the medium high heat to caramelize the onions and carrots a bit. About 5 minutes.
Add the Guinness and beef stock and bring to a boil.
Add the thyme and cover reducing the heat to a simmer.
Simmer for about 1 to 1 1/2 hours until the lamb is very tender. Add the potatoes for the last 40 minutes of cooking.
Season with salt and pepper and if the stew is too thin, take the reserved flour and 2 teaspoon of butter and mix them together with your hands. (Buerre Manie) and whisk in pea sized pieces until the thickness is to your liking. Make sure to allow stew to return to a full simmer for 2 to 3 minutes to let the four incorporate and thicken completely.