These make a wonderful addition to any summer salad or antipasto.
Fill a large bowl with cold water; juice the lemon into the water; set aside. Halve artichokes; if they have any pink choke, trim it from the center, leaving artichoke half intact. Transfer artichokes to acidulated water.
Bring a large saucepan of water to a boil over high heat. Drain artichokes, and add to boiling water. Reduce heat to medium; gently simmer artichokes until just tender, 5 to 7 minutes. Remove from boiling water, and drain; set aside.
Make vinaigrette: In a medium bowl, whisk shallot, mustard, and vinegar. While whisking, slowly add oil. Stir in tarragon; season with salt and pepper. In a large bowl, toss artichokes with 1/2 cup vinaigrette; transfer to a serving dish. Strip leaves from one sprig of thyme, and sprinkle on artichokes. Use remaining sprigs for garnish. Serve artichokes with remaining vinaigrette on the side.