In a large Dutch oven or heavy pot, heat the olive oil over medium-high heat. Season the lamb cubes generously with salt and black pepper. Working in batches, sear the lamb on all sides until browned, about 5-7 minutes per batch. Remove the browned lamb and set aside.
In the same pot, add the diced onion and sauté until softened, about 5 minutes. Stir in the minced garlic and cook for an additional minute until fragrant.
Add the carrots and celery to the pot, sautéing for another 5 minutes until they begin to soften.
Return the lamb to the pot, then pour in the beef broth and diced tomatoes. Add the rosemary, thyme, bay leaf, potatoes, and additional salt and pepper to taste. Stir well to combine.
Bring the stew to a simmer, then reduce heat to low, cover, and let it cook for 1.5 to 2 hours, or until the lamb is tender and can easily be shredded with a fork.
About 10 minutes before serving, stir in the green peas. If you prefer a thicker stew, mix 1 tablespoon of cornstarch with 2 tablespoons of cold water to create a slurry, and stir it into the stew, cooking for an additional 5 minutes.
Remove the bay leaf, adjust seasoning if necessary, and garnish with chopped fresh parsley before serving.