Heat oven to 400°F. Line a sheet pan with foil and spray with cooking spray. Toss 1 bunch asparagus spears with olive oil. Peel and slice sweet potatoes very thinly (1/8th inch). Toss with olive oil. Sprinkle with salt and pepper. Chop 1 1/2 c. walnuts, combine with 1 tbsp lemon zest, 1 tbsp maple syrup and 1 tsp dried dill. Spread each filet with 1 tbsp Dijon mustard. Press walnut mixture on top. Arrange asparagus and sweet potatoes on baking sheet and top with halibut. Bake at 400°F approx 15-20 minutes. Let stand for 5-7 minutes. Serve.