1. Whisk together the flour, 1 teaspoon of the salt, and the pepper and spread it out on a plate.
2. Dredge the pieces of lamb in the seasoned flour and shake off any excess.
3. Heat the oil in a large, heavy Dutch oven or deep saucepan over medium-high heat.
4. Add approximately ⅓ of the lamb to the pot, so as not to crowd it, and brown the pieces on all sides, 4 to 5 minutes total.
5. Remove the browned pieces to a plate. Repeat two more times until all the lamb has been browned, adding more oil to the pot if necessary. Set aside.
6. Add the onion to the pot and scrape up any browned bits stuck to the bottom of the pot with a wooden spoon. Season the onion with a pinch of salt and pepper. Sauté the onion over medium heat until softened, about 5 to 7 minutes. If the onion is sticking to the pan, add a tablespoon of water.
7. Add the garlic, tomato paste, rosemary, and thyme, and sauté an additional minute until fragrant.
8. Return the meat and any accumulated juices to the pot. Add the stout and the broth or water. The liquid should mostly cover the meat but not submerge it. If the liquid is too low, add another splash of broth. Taste the liquid (it will taste bitter, but don’t worry), and add more salt and pepper if needed.
9. Increase the heat to high and bring the stew to a boil. Then reduce the heat to low or medium-low and simmer the stew, uncovered, until the lamb is tender, about 30 minutes.
10. Add the carrots and potatoes and stir to combine. Increase the heat and bring the mixture back to a boil.
11. Partially cover the pot and turn the heat down to medium-low or low and simmer stew just until the vegetables are tender, 20 to 30 minutes.**
12. Add the peas and simmer just until the peas are heated through. Taste and adjust the seasoning, adding more salt or pepper as needed.
13. Garnish with additional chopped fresh rosemary if desired. Serve immediately.

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