Preheat oven to 350
In a small bowl, combine garlic, mint, oregano, 2 tbs olive oil, 1 tsp salt and pepper. Place lamb on a work surface and spread the herb paste evenly on the meat. Roll up the lamb and tie with kitchen string at 2 to 3 inch intervals.
In roasting pan over med-high heat, warm enough oil to coat bottom of the pan. Add the lamb and brown on all sides, 4 to 5 min. per side. Transfer the pan to the oven and roast, turning lamb every 20 min., until an instant-read thermoer inserted into the thickest part registers 130 for med. rare, about 1 hour. Transfer the lamb to a carving board, cover loosely with foil and let rest for 15 min.
Skim the fat from the pan and set over med. heat. Add the demi-glace sauce and stock and cook, stirring to scrape up the browned bits from the pan bottom. Strain the sauce into a saucepan, set over medium heat and cook until thickened, about 5 min. Whisk in the butter 1 tbs. at a time, season with salt and pepper.
Snip the strings and thinly slice the lamb across the grain.