Melt the butter in a small skillet over medium heat. Heap in the leeks. They should be crowded. The leeks will cook down as they melt; this will take about 6 - 8 minutes. You want to catch them when they just begin to get mushy, but when they still have a nice texture.
While leeks are cooking, prepare the scallops and make the vinaigrette. Remove muscle from the scallops if present, and season with salt and pepper. For the vinaigrette, chop the pistachios coarsely (I like to leave a few whole). Reserve just a bit of the chopped pistachios for garnish and add the rest to a bowl. Add the honey, mustard, the juice of 1/2 of the lemon (reserve the other half for later), salt and pepper, and whisk together. Drizzle in 1 1/2 to 2 Tablespoons olive oil. Taste and adjust seasonings.
When leeks are done, remove them from the pan and set aside. Wipe out the pan and heat the remaining olive oil over medium heat. Sear the scallops until they are cooked through and just barely firm, about 3 - 4 minutes per side. They should have a little color. Just before they are done, squeeze the reserved lemon half over them and swirl the pan around to cover the scallops with the lemon and pan juices.
For presentation, I spoon the leeks into small mounds (3 per plate) and put scallops on top, then add the greens in the middle of each plate. Drizzle the vinaigrette over each dish and sprinkle the reserved pistachios over to garnish.