Juicy Lamb & Purple Potato Shepherd's Pie with a Blackberry Demi Glace

The Dr. Seuss-ian purple mashed potato topping you see here consists of extras from the Basque potato tortilla that I made for last week's French Fridays with Dorie.  I first got a bee in my bonnet to make a shepherd's pie because Claudia from What's Cooking? recently made an Italian-style version of the dish, which got my tummy rumbling.  I also just happened to have eight braised lamb shoulder chops sitting around.

But why, you ask, when it's just Poppa Trix and me, would I ever need to make such an obscene amount of lamb all at once?  Why, indeed!