1. Bring a large saucepan of salted water to a boil. Fill a large bowl with ice water. Boil snap peas until bright green (15 to 20 seconds), then drain. Rinse with cold water and add to ice water.
2. Meanwhile, in another large saucepan, combine 1/4 cup liqueur, 1 tbsp. salt, the bay leaves, and 1 qt. water. Squeeze juice from lemon halves into pan and add halves as well. Bring to a boil, stir in shrimp, and remove from heat. Let stand until shrimp are cooked through, about 3 minutes. Drain, rinse shrimp with cold water, and add to ice water with peas. Let stand until cold, then drain peas and shrimp well.
3. Whisk remaining 1 1/2 tsp. liqueur, 3 tbsp. lemon juice, the oil, mayonnaise, remaining 3/4 tsp. salt, and the pepper in a large bowl. Add peas, shrimp, chervil, radishes, and fennel, tossing to coat.
4. Serve with lettuce for people to use as a base for their salads if they like.
*If you can't find chervil, use equal parts fresh tarragon and flat-leaf parsley.