Heat the vegetable oil in a large pot or Dutch oven over medium-high heat.
Add the cubed lamb meat and brown it on all sides. This should take about 5 minutes. Remove the browned meat from the pot and set it aside.
In the same pot, add the chopped onion and minced garlic. Sauté for a few minutes until the onions are translucent and fragrant.
Add the sliced carrots, diced potatoes, sliced leek (if using), and sliced celery to the pot. Cook for about 5 minutes, stirring occasionally.
Sprinkle the all-purpose flour over the vegetables and stir well to coat them. This will help thicken the stew.
Return the browned lamb meat to the pot and pour in the beef or vegetable broth and red wine (if using). Stir to combine.
Add the bay leaves and dried thyme to the pot. Season with salt and pepper to taste.
Bring the stew to a boil, then reduce the heat to low, cover, and simmer for about 1.5 to 2 hours, or until the lamb is tender and the vegetables are cooked through. Stir occasionally and add more broth if needed to maintain the desired consistency.
About 15 minutes before serving, add the frozen peas to the stew and stir. Cook until the peas are heated through.
Taste the stew and adjust the seasoning if necessary.
Remove the bay leaves before serving.
Garnish the lamb stew with chopped fresh parsley, if desired.Serve the delicious lamb stew hot and enjoy!