Prepare the grill and let it burn down to medium coals. Leave the coals piled in the center of the grate. Sprinkle the salmon all over with salt and let rest for about 25 minutes. Rinse and pat dry with paper towels. Brush with olive oil.

Add a handful of soaked chips to the coals, oil the grill rack, if necessary, and place the salmon, skin side down, on the grill toward the outside of the coals. Cover the grill and cook, without turning, until the salmon is opaque through, about 20 minutes (for 1-1/2-inch-thick fillets).

Meanwhile, put the potatoes in a pot of cold salted water and bring to a boil. Cook until tender, about 5 minutes. Lift out with a slotted spoon and spread on a baking sheet to cool. Return the water to a boil, add the peas, and cook just until bright green and tender, about 3 minutes. Drain well and scatter on the baking sheet with the potatoes to cool.

In a small bowl, whisk together the mustard, vinegar, the tarragon, if using, and salt and pepper to taste. Pour in the 1/2 cup olive oil while whisking to form an emulsion.

Put the peas and potatoes in a large bowl and add about 1/4 cup of the vinaigrette. Toss and taste for seasoning. Add more of the vinaigrette, if needed. Toss well with 1 tablespoon of the chives and divide among 4 plates. Pull the skin off each piece of salmon (reserve, if desired), then place on top of the salads. Dress the greens with another tablespoon of the vinaigrette and arrange around the salmon. Sprinkle the salads with the remaining 1 tablespoon of chives.