Bring 1/2 cup of water to boil in a large skillet. Add the spinach and cook until it just wilts, about 1 to 2 minutes. Drain well, cool to room temperature, and then squeeze out the excess moisture. Coarsely chop the spinach and place in a bowl. Mix in the garlic and pine nuts.
With a long, narrow knife, make numerous, deep slits into the flesh of the lamb. Stuff the spinach mixture into the slits. Preheat the oven to 425 degrees F. Place the lamb into a roasting pan. Brush with Dijon mustard; sprinkle with rosemary, salt and pepper. Roast 20 minutes. Reduce heat to 325 degrees F and cook lamb to desired degree of doneness, checking its progress with a meat thermometer. 75-90 minutes should make medium rare in the middle.