Step 1Bring water to boil in a 6-quart pot. Add 2 tablespoons of salt. Fill a large bowl with water and ice.
Step 2Pinch off the stem ends of the haricots verts. Add to the pot and cook until just tender, 1 to 2 minutes. The beans will turn bright green and retain a slight crunch. Drain and immediately plunge the beans into the ice water. This will set the color and stop the cooking. When the beans have cooled, remove and drain.
Step 3Heat the vegetable oil in a skillet over medium-high heat. Add the mushrooms and 1/4 teaspoon of salt. Cook the mushrooms until they have released their juices and are lightly browned, 4 to 6 minutes. Set aside to cool.
Step 4Whisk together the lemon juice, mustard, garlic, 1/4 teaspoon of salt and the pepper. Slowly whisk in the olive oil. Add the rosemary and thyme. Whisk until the dressing holds together, about 1 minute.
Step 5Toss together the haricots verts, the orange and yellow bell peppers, onion, mushrooms and the dressing in a large bowl. Add the salmon and mix gently. Serve on a bed of butter lettuce and garnish with thin slices of lemon.