For the Black Pepper-Sweet Mustard Sauce: Combine the mustards, honey, black pepper, thyme in a medium bowl and season with salt. Preheat oven to 375 degrees F. Heat a large ovenproof saute pan over high heat until smoking. Season each breast with salt and pepper to taste. Score the skin with a knife and sear the breast, skin-side down, until golden brown and fat has rendered, about 6 to 8 minutes. Remove most of the fat, turn the breasts over and place them in the oven and cook for 10 to 12 minutes for medium rare doneness, remove duck and brush liberally with the mustard glaze. Let duck rest for 10 minutes, slice each breast on the bias.
For the Soft Mascarpone Polenta: Heat olive oil in a medium saucepan over medium heat. Add the leeks and cook until very soft, add the asparagus, season with salt and pepper and cook for 5 minutes. Place water in a large saucepan with 2 tablespoons of salt and bring to a boil. Slowly whisk in the polenta, cook for 25 to 30 minutes, stirring every few minutes with a wooden spoon.
Remove polenta from heat, whisk in mascarpone, Manchego and leek/asparagus mixture, season with salt and pepper. Keep warm.