Fill a bowl with cold water; juice lemon into water; set aside. Trim artichoke tops; trim tough stems; halve lengthwise. Trim any pink choke from center, leaving half-intact. Transfer to acidulated water.
Bring a saucepan of water to a boil over high heat. Drain artichokes; add to boiling
water. Reduce heat to medium; gently simmer artichokes until just tender, 5 to 7 minutes. Drain; set aside.
Toss artichokes with 1/2 cup vinaigrette; transfer to a serving dish. Strip leaves from 1 thyme sprig. Sprinkle on artichokes; use remaining sprigs for garnish. Transfer remaining vinaigrette to a serving bowl; serve with artichokes.
1 shallot, finely chopped
2 tablespoons Dijon mustard
4 tablespoons white-wine vinegar
3 tablespoons freshly squeezed lemon juice
Coarse salt and freshly ground pepper
3/4 cup plus 2 tablespoons olive oil
1 tablespoon finely chopped tarragon leaves
In a bowl, whisk shallot, mustard, vinegar, lemon juice, salt, and pepper. Slowly add oil while whisking. Stir in tarragon.