How to Make It
Combine first 5 ingredients in a large zip-top plastic bag; seal and marinate in refrigerator overnight or up to 2 days.
Prepare grill to medium-high heat.
Remove duck from bag; sprinkle evenly with 1/4 teaspoon salt and 1/8 teaspoon pepper. Place duck on a grill rack coated with cooking spray, and grill 4 minutes on each side or until desired degree of doneness. Remove duck from grill; cover and let stand 10 minutes. Cut the duck across grain into thin slices.
While duck stands, heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add mushrooms, 1/8 teaspoon salt, and remaining 1/8 teaspoon pepper; sauté 10 minutes or until tender. Add asparagus; sauté 2 minutes. Combine remaining 1/8 teaspoon salt, vinegar, and truffle oil, stirring with a whisk. Arrange about 1/2 cup mushroom mixture in center of each of 4 plates. Top each serving with 1 sliced duck breast half. Drizzle about 1 1/2 teaspoons vinaigrette over each serving.