Because I tend to find lamb to be a bit gamey, I cut it with equal parts ground beef; if you love lamb, omit the beef and use 1 1/2 pounds lamb instead (this is what the original recipe calls for). If you don't want to buy both rosemary and thyme, choose one and use more of it to make up the difference.
Like most casseroles, this recipe is great as leftovers (the leftovers even fare well frozen).
To make it ahead, freeze the entire assembled casserole dish (tightly-wrapped in tinfoil), then, the night before you'd like to eat it, defrost overnight in the fridge before baking as normal.