In a large saucepan, cover the potatoes with water and bring to a boil. Reduce heat, cover, and simmer for about 20 minutes, or until potatoes are tender. Set aside.
In a large skillet, heat the olive oil over medium heat. Add the onion, carrot, and mushrooms, and cook, stirring, until lightly browned. Add the ground lamb and cook, stirring and breaking up, until lightly browned.
Blend 2 tablespoons of the broth with the flour, whisking until smooth. Stir into the pan along with the remaining broth. Bring to a simmer, stirring constantly. Add the bay leaf, Worcestershire sauce, and tomato paste. Cover and simmer for 45 minutes to 1 hour, stirring occasionally. Uncover and simmer for about 5 to 10 minutes longer, until slightly reduced and thickened. Stir in the dried tarragon.
Heat the oven to 375°.
Heat the potatoes until hot. Drain well and mash with the butter, milk, and parsley.
Spoon the meat mixture into an 8-inch square pan or deep pie pan. Top with the mashed potatoes, spreading to cover the meat mixture.
Bake for 25 to 30 minutes, until nicely browned.
Serves 4 to 6
Calories per serving (based on 6 servings): 432
Fat per serving (based on 6 servings): 17.6