3 tbsp red wine vinegar
1 extra-virgin olive oil, divded
1 tbsp Dijon mustard
3/8 tsp salt
3/8 tsp pepper
4 5- to 6-ounce boneless, skinless salmon fillets (about 1 inch thick)
12 oz white mushrooms wiped clean, stem ends trimmed
12 oz stemmed spinach, rinsed and dried
1/2 cup loosely packed picked-fresh dill

For the vinaigrette, combine the vinegar, 1/2 cup plus 1 tablespoon of extra virgin olive oil, mustard, 3/8 teaspoon salt and 3/8 teaspoon pepper in a small bowl and whisk with a fork; set aside. 

For the salmon, preheat the oven to 350°F. Line a baking sheet with foil. On the baking sheet, sprinkle the fillets on both sides with salt and pepper and let stand 5 minutes to allow the seasonings to absorb. Coat the fillets on both sides with about 1 teaspoon extra virgin olive oil each. Bake the salmon until medium-rare and just translucent in the center, 13 to 15 minutes. Let cool to room temperature, then refrigerate if not using immediately. 

Cut a thin slice off the top of each mushroom cap to stabilize it. Set the mushrooms, cut side down on the cutting board and cut into 1/4-inch-thick slices. 

In a large bowl, combine the spinach, sliced mushrooms, and dill. Give the vinaigrette a stir with a fork and add to the bowl. Break the salmon into pieces and add to the bowl. Season with a pinch more salt and pepper. Toss the salad very gently until everything is lightly coated with vinaigrette.