Dust the chopped lamb pieces in some flour and shake to remove the excess.
Heat several tablespoons of oil in a Dutch oven or large pot. Brown the lamb pieces over medium high heat on the stove top for several minutes. Transfer to a dish.
Add the diced onion, carrot, celery and garlic to the Dutch Oven (or pot) and cook until tender (3 – 4 minutes). Add lamb pieces back into the pot with the veggies, then add the wine and beef stock, thyme and bay leaves. Bring the liquid to a soft boil, reduce the heat, cover the pot with a lid and let simmer on low heat for 1.5 – 2 hours or until the meat falls apart and is easy to shred. If necessary, add more stock to the pot to ensure the ingredients are fully covered by the cooking liquid.
About 10 minutes before the lamb is fully cooked, add the sliced mushrooms and peas (optional) to the pot. Add salt and pepper to taste.
Strain the meat and veggies from the cooking liquid. Separate the meat from the veggies and shred it using a fork to ‘mash’ the meat fibers.
Place the shredded lamb and cooked veggies into a pie or casserole dish, filling the dish almost to the top.
Pre-heat the oven to 180 C (360 F).
Return the cooking liquid to the stovetop and let simmer until the sauce reduces to about ¾ of the original volume. If required, add a bit of cornflour paste to the liquid to thicken it further. Add the sauce on top of the lamb mixture.
Remove the frozen puff pastry sheet from the freezer and let it thaw for several minutes. Then place it on top of the pie or casserole dish (containing the lamb mixture). If necessary, trim the outer edges of the pie so that the pastry fits neatly around the rim of the dish. Brush the dough lightly with an egg wash (1 egg mixed with a little water).
Cook for 30 minutes or until the crust turns a golden brown.