It absolutely must not contain tomatoes. That is not shepherd's pie.
I fry the lamb mince with onion, tiny chopped up carrots and caugettes, loads of garlic (not authentic but lovely), then add flour to thicken it and stock or water, worcestershire sauce, salt and pepper.
I don't like dry shepherd's pie, so I make a lot of gravy, then put the mince mixture into the pie dish with a slotted spoon so I'm left with about half the gravy, reserve that, top the pie with mash (a mix of sweet potato and potato is really fab), then, after it has cooked and browned in the oven, I serve it with the reserved gravy available in a gravy boat to pour all over it
The mash really shouldn't have cheese in it, if you want it authentic, although I agree it adds a certain yummy rishness