1. Combine the oil, rosemary and garlic in a large zipper-lock bag. Add the lamb, then seal the bag, forcing out the excess air. Marniate in refrigerator for 8-24 hours, turning bag several times.
2. Preheat oven to 400 degrees.
To make the Berbere paste: Stir together 1 tablespoon of the berbere, the mustard, yolk and bread crumbs in a small bowl, then add 1 to 2 tablespoons of the wine, just enough to make a paste. Refrigerate until ready to use.
3. Remove the lamb from the marinade, pat dry and discard the marinade. Heat a dry, large, heavy saute pan over high heat until very hot. Reduce the heat to medium-high and brown the lamb, one rack at a time, about 3 minutes per side. Transfer, fat side up, to a large roasting pan.
4. Smear the berbere paste on the fat side of the lamb. Roast until instant-read thermometer inserted into the center of the rack reads 125-degrees — about 18-20 minutes.
5. While lamb is roasting, toast the remaining two tablespoons berbere in a small heavy saucepan over low heat, stirring constantly, until very fragrant, about 1 minute. Add the chicken stock and 1 tablespoon wine and bring to a boil, then boil until reduced to the consistency of a sauce. Remove from heat and whisk in the butter bit by bit until incorporated.
6. Cut the lamb into chops and serve the sauce on the side.