Pat the 2 ½ lb boneless lamb shoulder or stew meat pieces thoroughly dry with paper towels. Season all over with the 1 ½ tsp kosher salt and ½ tsp black pepper freshly ground. Drying the meat is essential — wet lamb will steam instead of sear.

Heat the olive oil in a large Dutch oven or heavy-bottomed pot over medium-high heat until shimmering. Working in batches (don't crowd the pan), sear the 2 ½ lb boneless lamb shoulder or stew meat pieces for 3–4 minutes per side until deeply browned. Transfer the browned lamb to a plate and set aside.

Reduce the heat to medium. Add the 1 large onion and 2 stalks celery to the same pot (with the drippings) and cook, stirring occasionally, for 4–5 minutes until softened and lightly golden.

Add the 3 cloves garlic and 2 tbsp tomato paste and stir for 1 minute until fragrant and the paste turns a slightly darker, brick-red color.

Sprinkle the 2 tbsp all-purpose flour for thickening evenly over the onion mixture and stir well to coat, cooking for 1 minute.

Pour in the lamb stock and 1 cup water to cover, scraping up any browned bits from the bottom of the pot with a wooden spoon — those bits are pure flavor. Return the browned 2 ½ lb boneless lamb shoulder or stew meat (along with any resting juices) to the pot.

Add the 2 whole bay leaves, 4 sprigs fresh thyme sprigs, and 1 sprig fresh rosemary (if using). Bring the stew to a boil over high heat, then reduce the heat to low. Cover with a lid slightly ajar and simmer gently for 45 minutes.

Add the 3 medium Yukon Gold potatoes peeled and cut into 1.5-inch chunks, 3 medium carrots peeled and cut into 1-inch pieces, and 1 medium parsnip peeled and cut into 1-inch pieces (if using) to the pot. Stir to submerge them in the broth, adding a splash more 1 cup water to cover if needed to just cover. Continue to simmer, partially covered, for another 35–40 minutes, until the 2 ½ lb boneless lamb shoulder or stew meat is fall-apart tender and the vegetables are completely cooked through.

Remove and discard the 2 whole bay leaves, 4 sprigs fresh thyme sprigs, and 1 sprig fresh rosemary sprig. Taste the broth and adjust seasoning with 1 ½ tsp kosher salt and ½ tsp black pepper freshly ground as needed. If you'd like a thicker stew, use the back of a spoon to lightly mash a few potato chunks against the side of the pot and stir them in.

Ladle into deep bowls, garnish generously with fresh fresh flat-leaf parsley roughly chopped, and serve with crusty soda bread or Irish brown bread for dipping.