In a small bowl, whisk together the honey and mustard to form a glaze. Set aside.
Prepare an ice-water bath in a large bowl and bring a medium pot of salted water to a boil. Blanch the asparagus until tender, 6 minutes. Drain and refresh them in the ice water to preserve their color. Drain again and set aside.
Heat the lentils over a low flame and keep warm.
Spray a grill or grill pan with nonstick spray and preheat until it's very hot (if using a grill pan, heat for at least 4 minutes). Dry the fillets and season the flesh sides with 1/2 teaspoon of the salt and 1/4 teaspoon of the pepper, then brush both sides with the olive oil. Place the fish, skin side down, on the grill and cook for 4 minutes. Flip the fish and brush the skin side with the reserved honey-mustard glaze. Continue to cook for 2 minutes. Hold the cooked trout aside.
Melt 3 tablespoons of the butter in a large saute pan over medium heat. Add the walnuts and toast for about 3 minutes, shaking the pan often to prevent burning. Season the nuts with 1/4 teaspoon salt, 1/4 teaspoon of black pepper, and 3 tablespoons of the balsamic vinegar. Cook for 3 to 5 minutes, until the vinegar has reduced to a thick syrup that coats the nuts. Add the remaining tablespoon of butter and stir for an additional minute. Turn the flame down to low and add the asparagus.
In a small saucepan, bring the remaining 1/2 cup of balsamic vinegar to a boil. Cook for about 3 minutes, until it has reduced down to 2 tablespoons.
Turn the flame off under the asparagus-walnut mixture and add the baby spinach along with a drizzle of the balsamic glaze. Gently toss to combine, and allow the heat of the pan to wilt the spinach.
Plate the salad by creating a small bed of the spinach-asparagus mixture in the center of the plate, surrounded by 3 dollops of warm lentils. Place 2 trout fillets, skin side up, on top of the spinach mixture. Drizzle with the balsamic glaze.
Recipe courtesy Keith McNally, Riad Nasr and Lee Hanson, The Balthazar Cookbook, Clarkson Potter, 2003