Spinach can be made 1 day ahead of assembling shepherd's pie and chilled, covered. Bring to room temperature before proceeding. Lamb shanks can be made 1 day ahead of assembling shepherd's pie and chilled, covered. Bring to room temperature before proceeding. Mashed potatoes can be made 1 day ahead of assembling shepherd's pie and chilled, covered. Bring to room temperature before proceeding. Shepherd's pie can be assembled (before being drizzled with melted butter) 1 day ahead and chilled, covered. Let stand at room temperature 1 hour before drizzling with butter and baking.