In large bowl, combine rosemary, 3 teaspoons garlic, lemon zest, 1/4 cup cooking wine and olive oil. Add racks of lamb, meat-side up, and cover bowl. Marinate 6-24 hours. After marinating, heat oven to 400 F. Transfer racks of lamb to heated, oven-proof saute pan on stovetop and sear each side 2 minutes until golden brown.
Transfer oven-proof saute pan with lamb to heated oven and cook approximately 10 minutes, or until lamb reaches 135 F in center. Remove pan from oven and take lamb out of pan to rest.
To make Marsala Mushroom Sauce: In same pan, melt butter over medium heat; add mushrooms and saute. Add shallots and remaining garlic, cooking until fragrant. Deglaze with remaining cooking wine. Add beef stock and whisk in Dijon mustard. Season with salt and pepper, to taste, and cook 2 minutes to reduce and thicken.
Carve racks of lamb by cutting between ribs. Serve drizzled with Marsala Mushroom Sauce. Garnish with parsley.
