Preheat oven to 350°F
In medium high skillet, finely chop and brown ground lamb
Add onion, celery, carrots, mushrooms, minced garlic, parsley, rosemary and salt and pepper. Stir well and cook for about 5 minutes until the veggies become tender
Add flour, stir and cook for an additional 2-3 minutes to cook off the flour
Slowly add broth and stir. Add enough to desired consistency. Simmer for 3-5 minutes and remove from the heat.
Coat baking dish with olive oil. Layer the dish with a base of potatoes, half the filling, a layer of potatoes, the rest of the filling, and top with a final layer of potatoes
Cover with foil and bake for 50 minutes. Remove from oven, uncover, brush top with olive oil and bake for another 20 minutes uncovered.
You may need to place under broiler to get the potatoes crisp.
Remove from oven and let cool for 10 minutes before slicing and serving.