In saucepan with steamer basket, add water just until it reaches the bottom of basket and bring to a boil. Cut potatoes into 1 1/2 inch chunks, uniform in size, and place in steamer basket. Cover and steam over simmering water for 15 to 20 minutes, or just until they are still firm but can be easily pierced with a fork. Add sugar snap peas to potatoes during the last minute or two of cooking. You want them bright green but still crisp. Drain potatoes and peas and set aside.

In large bowl, combine lemon juice, Dijon mustard, garlic, and pepper. Whisk to blend. Slowly whisk in olive oil until mixture is creamy and emulsified. Add a pinch of salt, if you wish. Whisk in yogurt and dill. Add a splash of water if dressing seems too thick. Fold half of the dressing into potatoes and peas, and gently toss to coat. Add more salt and pepper, to taste, if you wish.

Arrange greens on a serving platter. Top with potatoes and peas and tuck pieces of salmon throughout. Drizzle with remaining dressing and scatter fresh tarragon, chives, and capers overtop. Splash with added lemon juice if you wish an additional hint of tartness.