Step 1Cook haricots verts in large pot salted water, uncovered, until just tender, about 5 to 7 minutes. Drain immediately, hold briefly under running water to stop cooking, then spread on cloth towel to drain. (Can be done several hours ahead; do not refrigerate.)
Step 2Whisk together mustard, vinegar, 1/2 yolk and 1/2 teaspoon salt in large bowl, mixing well. Slowly add walnut and canola oils, whisking until combined. Whisk in pepper. (Can be made several hours ahead and refrigerated.)
Step 3To serve, place lettuces in bowl. Toss with 1/2 vinaigrette until well coated. Adjust seasoning. Put haricots verts in another bowl and toss with remaining vinaigrette; arrange on greens. Garnish with tomato. Serve immediately.