You can steam the artichokes ahead of time and refrigerate, but they should be at room temperature before serving. You can also make the lemon-garlic sauce ahead, but it may stiffen up as it sits in the refrigerator. Save some of the potato cooking water and stir a couple teaspoons of it into the sauce to loosen it up before serving.
1. In small pot of boiling water, cook potato until tender, about 10 minutes. Add garlic during last 2 minutes of cooking. Drain, reserving 2 tablespoons of cooking liquid.
2. Transfer potato and garlic to large bowl and mash until smooth. Add 2 tablespoons of lemon juice, mayonnaise, sour cream, salt, cayenne, and reserved cooking liquid.
3. Meanwhile, trim off tough leaves from artichoke. Trim off tough end of stem, then peel remaining stem.
4. In large saucepan, with a steamer rack, combine 3 cups of water, remaining 2 tablespoons lemon juice, rosemary, and thyme.
5. Set artichokes on rack and bring water to a boil over high heat. Cover and and cook 20 to 25 minutes until artichokes are tender. Serve artichokes warm or at room temperature with lemon-garlic sauce.