Rinse the clams under cold running water to remove any outer grit. Place them in a large kettle that has a tight-fitting lid, cover the kettle, and cook the clams over medium-high heat for 6 to 7 minutes, shaking the pot occasionally. With a slotted spoon, remove the opened clams from the kettle, reserving the liquid, and set the clams aside to cool. If any clams have not opened, steam them 1 to 2 minutes longer, discarding those that still do not open. Strain the clam liquid through several thicknesses of cheesecloth, and set it aside. When the clams are cool enough to handle, remove and reserve the meat, discarding the shells.

Cut the potatoes lengthwise into quarters. Steam them over boiling water for 8 to 10 minutes or until the potatoes are just tender.

While the potatoes are steaming, prepare the dressing by adding all of the dressing ingredients to a small bowl or a jar that has a tight-fitting lid and whisking or shaking them until they are well combined.

When the potatoes are done, transfer them to a large bowl, add about half the dressing, and toss the ingredients well. Set the bowl aside until the potatoes have cooled to lukewarm.

When the potatoes have cooled add the reserved clams, red bell pepper, celery, and onion to the bowl. Drizzle the remaining dressing over the salad, and toss the salad.

To serve the salad, if desired line a large platter or 4 individual plates with the greens. Arrange slices of cucumber and tomato along the edge of the platter or plates, and heap the salad in the middle.