Make the spinach stuffing by sauteing 1/4 cup onions and garlic in the oil and butter. Add spinach and saute until wilted. Adjust seasoning with salt and pepper. Cool and combine with breadcrumbs and feta cheese.
Make the sun-dried tomato mixture by soaking the tomatoes to soften, drain and chop finely. Saute remaining 1/4 cup onions over medium high heat until translucent. Add chopped tomatoes, heat thoroughly. Adjust seasoning with salt and pepper.
Preheat the oven to 375 degrees F.
Lay out loin, meat side up, trim away excess fat and sinew. Lay spinach/feta mixture between the 2 loins. Roll over the right flap and tightly pack it around the left loin. Roll the newly formed double loin over the left flap, cut off excess. Tie the roast tightly. Use a steel or wooden spoon to create a hole down the entire length of the lamb loin on each side of the stuffing. Using a pastry bag, pipe the tomato mixture into each hole. Season with salt and pepper. Brown over medium high heat. Roast for approximately 30 minutes. Check doneness with meat thermometer -- 120 degrees.
Recipe courtesy of Thomas Hannum, CEC, Hotel Dupont, Wilmington, DE