Preheat oven to 240°C (220°C fan-forced).
Place potatoes in a large saucepan of cold salted water. Place over high heat and bring to a boil. Cook, for 1 to 2 minutes, or until almost cooked. Try not to overcook. Drain gently to avoid breaking them up, and set aside to dry.
Heat oil in a large, deep-sided, heavy-based ovenproof pan over medium-high heat. Add the onion, celery and silverbeet stems, cook, stirring for 4-6 minutes, or until just softened. Add garlic and cook for a further 2 minutes.
Add lamb mince, dried mint and ground cumin. Season with salt and pepper and cook for 4-6 minutes, breaking up with a wooden spoon, or until lamb is browned. Sprinkle over flour and stir. Add stock and bring to a simmer. Reduce heat to low and cook, uncovered, for 5 minutes, or until the sauce has reduced slightly.
Add silverbeet leaves, pine nuts, fetta and lemon rind, and stir to combine. Stir in parsley and dill.
Arrange potato slices on top, overlapping slightly. Brush with melted butter and season with salt flakes. Place on a baking tray and bake for 30 minutes, or until golden brown.
Set lamb mince pie aside for 5 minutes. Serve with lemon wedges.
