Store leftovers in an airtight container in the fridge for up to 4 days. Or, in the freezer, for up to 3 months. To reheat, transfer portioned out leftovers to an ovenproof dish and tent with foil. Heat at 325°F/163°C until lentils and potato-cauliflower topping are both warmed through. If frozen, I recommend allowing the shepherd’s pie to thaw before reheating (otherwise, the potato-cauliflower topping can be a bit icy and lend watery results).