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.