Preheat oven 350 degrees F.
Debone the lamb racks and remove any exterior fat. Cut up the bones and reserve for the lamb stock.
Sear the lamb fillets in a pan with olive oil for 2 to 3 minutes, until lightly browned on all sides. Season with salt and pepper and sprinkle with thyme. Set aside.
Stem, wash and pat dry the spinach. Melt the butter in a pan. Add the shallots and garlic and cook until browned. Add the spinach and cook for 1 to 2 minutes. Season with salt and pepper. Reserve.
Spread 1 sheet of the phyllo dough out onto a flat surface. Fold in half and brush with butter. Wrap the lamb fillets in prosciutto. Place the 1 lamb fillet on the phyllo and cover with 1/2 of the spinach. Wrap with the phyllo and place on a baking sheet. Repeat with the rest of the ingredients. Cook for 12 to 15 minutes.
Pour a little lamb stock on each plate before serving.
Put the bones in a saucepan and brown lightly. Add the onion, chopped carrot and the bouquet garni. Cover with water and season with salt and pepper. Cook slowly, uncovered, for 1 hour, skimming and degreasing from time to time.
Strain through a sieve or a fine cloth.
Recipe courtesy Chateau Frontenac, Quebec