Haha, I was thinking the same thing - I have never followed a recipe for shepherd's pie, ever.
Go the roast lamb...
Definitely this ^
Tonight we're having roast lamb and I will always do (depending on leftovers) a shepherd's pie the following night.
I basically finely chop the leftover lamb, fry with an onion, add fresh rosemary (garlic if you wish, this is one dish I don't add garlic too) and make a gravy using beef stock, tomato paste, tinned tomatoes and a good grinding of salt & pepper.
Put in a casserole dish, allow the mixture to cool a little and top with mashed potato. I put shaved Parmesan on top of the potato, another good grinding of salt & pepper and a knob of butter and bake until the top is brown and crispy.
I let it rest a bit once cooked so I can cut it into nicer slices (it's too sloppy if you cut it up straight out of the oven).
So, plan ahead. If your guests are coming Monday night, have a roast lamb Sunday (you'll need a big leg) and use the leftovers for a 'proper' shepherd's pie.
(I believe some traditionalists mince the lamb in an old fashioned mincer).