• Scrub the potatoes. Cut the potatoes into ½-inch pieces.
• Trim the root end from the leek; cut the leek in half lengthwise, then crosswise into ¼-inch-thick half-moons. Soak the half-moons in a medium bowl of cold water, stirring occasionally to loosen any dirt, and allow the dirt to settle at the bottom. Skim the leek from the surface of the water.
• Trim the stem ends from the green beans if needed; cut the beans into ½-inch lengths. 

In a large sauce pot, add the potatoes and leek and enough generously salted water to cover by 2 inches. Bring to a boil, reduce to a simmer, and cook until tender, 5 to 7 minutes, adding the green beans during the last 2 minutes of cooking. Drain and transfer to a large bowl.

Stir in half the “creamy” mustard vinaigrette, adding more if desired, and season to taste with salt and pepper. While the vegetables are cooking, prepare the pistachio-herb dressing.

• Finely chop the pistachios.
• Remove the stems from the caper berries; coarsely chop the berries.
• Coarsely chop the dill.
• Strip the parsley leaves from the stems; coarsely chop the leaves.

In a small bowl, stir together the pistachios, caper berries, dill, parsley, and 2 to 4 tablespoons oil; season to taste with salt and pepper.

For fish (salmon and snapper):

• Pat the fish dry with a paper towel; season lightly with salt and pepper. 

In a large frying pan over medium-high heat, warm 1 to 2 tablespoons oil until hot but not smoking. Add the fish (skin side down for the snapper) and cook until lightly browned (and the snapper skin is crisp), 4 to 5 minutes. Turn and cook until the flesh is opaque and flaky, 2 to 4 minutes.

For scallops:

• Rinse and drain the scallops. Pat dry on a paper-towel-lined plate. Season lightly with salt.

In a large frying pan over high heat, warm 1 to 2 tablespoons oil until hot but not smoking. Add the scallops and cook without turning until golden brown on one side and starting to turn opaque, 2 to 4 minutes. Turn and continue cooking for about 30 seconds for translucent or 1 to 2 minutes for opaque.

Transfer the seafood and potato-leek salad to individual plates. Top the seafood with the pistachio-herb dressing and serve any remaining mustard vinaigrette on the side.