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1
Fill a large bowl with cold water and ice cubes. Trim the ends of the haricots verts, if desired.
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2
Bring a large pot of lightly salted water to a boil over high heat. Add the haricots verts and cook for about 1 minute, just until bright green; use tongs to transfer the beans to the ice-water bath to stop their cooking. Transfer them to a clean, dry dish towel, wrap them up and refrigerate until ready to use.
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3
While the haricots verts are cooking, dry-toast the walnuts in a small skillet over medium-low heat for 3 to 4 minutes, shaking or stirring to make sure they do not burn. Remove from the heat; when the nuts have cooled slightly, chop them coarsely and place half of them in a large bowl.
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4
Finely grate the zest of 2 lemons (there should be 1 1/2 to 2 tablespoons), then cut the lemons in half and juice 3 halves (there should be about 3 tablespoons); reserve the remaining 1/2 lemon for another use.
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5
Combine the egg yolk, mustard and 2 tablespoons of the lemon juice in a medium mixing bowl or 2-cup glass measuring cup. Slowly whisk in the oil until the mixture is emulsified and quite thick. Add up to 2 tablespoons of lemon juice; add a tablespoon or two of water if a thinner consistency is desired. Season to taste with salt and pepper.
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6
Use a Microplane grater or zester to pulp the garlic cloves, working over the large bowl to catch any juices; then add the garlic pulp. Cut the cucumber into 1/2-inch dice (there should be about 2 1/2 cups) and add it to the bowl, along with the crabmeat and the lemon zest. Add the dressing and toss gently to combine.
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7
Wash and dry the lettuce and watercress leaves, discarding any tough watercress stems; tear the lettuce into bite-size pieces. Place the greens in a mixing bowl and drizzle with the walnut oil and the remaining 1 tablespoon of lemon juice.
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8
Distribute evenly among individual plates, then divide the haricots verts evenly and place them neatly on the greens; drizzle them with oil, if desired. Divide the dressed cucumber-crab mixture among the salads. Sprinkle the remaining chopped walnuts over the salads. Serve immediately.
