- 1/2 cup white wine vinegar
- 1/2 cup coarsely chopped fresh dill
- 2 tablespoons Dijon mustard
- 1 teaspoon kosher salt plus more for seasoning
- 1 teaspoon freshly ground black pepper plus more for seasoning
- 12 small red-skinned potatoes or new potatoes (about 1 pound)
- 2 cups (heaping) haricots verts (about 6 ounces), trimmed
- 1 cup fresh shelled (or frozen, thawed) peas
- 3 1 1/2-pound live lobsters or 3 cups cooked lobster meat
- 2 cups cherry tomatoes, halved
- 6 small red radishes, trimmed, quartered
Combine first 4 ingredients, 1 tsp. salt, and 1 tsp. pepper in a medium jar with a lid and shake to combine. Let stand until flavors meld, about 5 minutes. Add oil and shake vigorously to blend well. DO AHEAD: Dressing can be made 3 days ahead. Cover and chill. Shake just before using.
Place a metal steamer basket in a large pot. Add water to a depth of 2 inches. Bring water to a boil.
Place potatoes in steamer basket; cover and cook until tender, 15–20 minutes. Transfer potatoes to a plate and let cool. Continue with the following ingredients, maintaining water at a depth of 2 inches throughout and returning water to a boil between batches: Cook haricots verts until just tender and bright green, 4–5 minutes; transfer to a large bowl of ice water and let cool. Cook peas until tender, about 1 minute; transfer to large bowl of ice water with beans and let cool. Using a slotted spoon, transfer cooled vegetables to another plate and pat dry. Add eggs to pot and cook until hard-boiled, 12 minutes; add eggs to same bowl of ice water; let cool.
If using live lobsters, add to pot, cover tightly, and steam until bright red and cooked through, about 10 minutes. Immediately transfer to another large bowl of ice water and let cool for 5 minutes. Remove lobsters from ice water.
Crack lobster shells and remove meat; coarsely chop. Discard shells. Cut potatoes into 1/2-inch rounds. Peel eggs; cut in half. DO AHEAD: Can be made 8 hours ahead. Cover ingredients separately and chill.
Lay mixed greens on a large serving platter. Arrange potatoes, haricots verts, peas, eggs, lobster meat, tomatoes, and radishes over however you choose. Season with salt and pepper. Serve with dressing.