Heat olive oil over medium heat in a heavy saute pan. Add leeks, shallots and white part of green onions. Cook until leeks are tender, 6-8 minutes. In a heavy 12-inch skillet, heat butter over medium-high heat. Season duck breasts with salt and pepper, then add to skillet; cook 4 minutes on each side. Place duck on a plate and tent with foil to keep warm. Add wine to same skillet, scraping up browned bits; cook until reduced to 1/2 cup. Add cream and boil 2-3 minutes to thicken. Season with salt and pepper. Stir in mustard; heat through. To serve, puddle some sauce onto 4 plates. Slice duck breasts and arrange over sauce. Serve with leek mixture alongside and garnish with reserved green onion tops.