Preheat oven to 220°C. Lay four 30cm squares of baking paper on 2 baking trays.
Place a salmon fillet on each, then top with lemon slices and a thyme sprig. Drizzle with 1 teaspoon of oil and season. Fold the edges of the paper together to form well-sealed parcels. Bake for 15 minutes until cooked through.
While salmon is baking, whisk the mustard, tarragon, vinegar, sugar and remaining oil in a jug for the dressing, and season.
Blanch the asparagus for 2 minutes in a pan of lightly salted boiling water until tender. Drain well, then keep warm.
Arrange the salmon and asparagus on plates, add slice of lemon , drizzle with the tarragon dressing and top with a sprig of thyme to serve.