Preheat the oven to 325 degrees.

Mix together the lamb, garlic, onion, and rosemary. Put through a meat grinder twice or chop in a food processor until fine. Melt the butter in a skillet and stir in the flour. Cook for a few minutes until smooth and blended. Slowly add the beef broth, whisking to avoid lumping. Cook, stirring, until the gravy is thickened, at least 5 minutes to get rid of the raw flour taste. Add the lamb mixture, stir to blend, and season with salt and pepper to taste.

Spoon into a 1½-quart casserole or a deep pie pan. Spread the mashed potatoes on top and cover evenly to the edge of the casserole. Make a crisscross design with a fork.

Bake for 45 to 50 minutes, until the meat is bubbling hot and the potatoes are browned.