Season the Lamb: Generously sprinkle the lamb chunks with salt and pepper, and let them sit for about 5 minutes to absorb the seasoning.
Combine Ingredients in a Pot: In a large Dutch oven or a heavy soup pot, place the seasoned lamb, potato, carrots, turnip, leek, barley, and celery. Add 2 teaspoons of salt and ½ teaspoon of pepper. Drop in the tied bundle of thyme sprigs and the bay leaf to infuse the stew with flavor.
Add Water and Simmer: Pour in 6 cups of water and bring the mixture to a boil over medium-high heat. Once boiling, lower the heat to a gentle simmer. Skim off any foam or impurities that may rise to the surface. Let the stew cook slowly until the lamb is tender and starts to fall apart, which will take about 1 to 1½ hours. If the stew becomes too thick during cooking, add a little more water to adjust the consistency.
Incorporate the Kale: About 15 minutes before the stew is fully cooked, stir in the kale. This will allow it to soften and blend into the stew without overcooking.
Taste and Serve: Taste the stew and adjust the seasoning with additional salt and pepper if needed. Ladle the hot stew into individual bowls, garnishing with fresh parsley and celery leaves. For an optional tangy touch, drizzle with cider or malt vinegar before serving.