Lamb Roast with Spinach Stuffing

1  leg of lamb, boned, butterflied
¼ c  olive oil
¾ c  chopped onion
½ c  chopped celery
3 cloves garlic, minced
10 oz chopped spinach, thawed and drained
2 medium eggs, lightly beaten
¼ c chopped parsley
3 T thinly sliced fresh basil
1 tsp crushed dried rosemary
¼ tsp crushed dried marjoram
¼ tsp black pepper
6 c  plain croutons
½ c  water
¼ c  grated Parmesan cheese
Fresh mint sprigs, for garnish

 Preheat oven to 325°F.
Heat oil in a large skillet over medium; add onion, celery, and garlic. Cook until softened, about 5 minutes, stirring. Set aside.
Meanwhile, mix together spinach, eggs, parsley, basil, rosemary, marjoram, and pepper in a large bowl. Add onion mixture; mix well. Add croutons, water, and cheese; mix gently. Set aside.
Pound lamb to an even thickness; spread stuffing over the top. Roll up meat, jellyroll style; tie. Place lamb seam side down on a rack in a shallow roasting pan. Bake 2 hours. Remove from oven; allow to rest 15 minutes. Carve and serve with fresh mint.