Whisk together yogurt, mustard, olive oil, vinegar, shallot, 1/4 teaspoon of the salt and the pepper; set aside.
Fill a large, deep skillet half-full with water and add remaining 1 tablespoon salt. Bring to a boil over high heat, then add green beans to pan. Cook beans 5 minutes or until crisp-tender. Using a slotted spoon, remove green beans from skillet and rinse under cold water. Place in a serving bowl with lettuce and tomatoes.
Return water in skillet to a boil and add salmon and lemon juice and zest to pan. Cover pan, then remove from heat and let stand for 15 minutes or until salmon is opaque throughout. Remove from pan and drain salmon on a paper-towel-lined plate.
Dress lettuce mixture with half of prepared dressing; divide among 4 plates. Place 1 salmon fillet over top and drizzle with remaining dressing; serve immediately.