Preheat the oven to 425°F. In a small bowl, place the mustard. Whisk in the vinegar. Gradually whisk in 2½ tablespoons oil. Mix in the shallot and thyme. Season the vinaigrette to taste with salt and pepper. Brush a small baking dish with oil. Place the salmon in the dish, skin side down. Spoon ½ of the vinaigrette over the salmon. Let marinate for 15–20 minutes at room temperature, or refrigerate for up to 1 hour.
Roast the salmon until almost cooked through, about 15 minutes. Let it rest while preparing the salad.
In a bowl, toss the lettuces with the remaining vinaigrette. Divide the salad between 2 plates. Cut the salmon in half and put one piece alongside the salad on each plate. Serve right away.