To prepare dressing, combine first 6 ingredients in a bowl, stirring well with a whisk until smooth. Set aside.
To prepare steak, combine onions, 2 tablespoons soy sauce, 2 teaspoons vinegar, sugar, and minced garlic in a large zip-top plastic bag. Add steak to bag; seal. Marinate in refrigerator 1 hour, turning occasionally.
Remove steak from bag. Place steak on grill rack coated with cooking spray; cook 5 minutes on each side or until desired degree of doneness. Let stand 5 minutes. Cut steak diagonally across grain into thin slices. Place steak in a medium bowl. Drizzle with half of dressing; toss well.
Place salad greens in a large bowl. Drizzle with remaining dressing; toss well. Place about 1 cup salad mixture on each of 4 plates. Top each serving with 3 ounces steak, 4 tomato wedges, and one quarter of cucumber slices. Sprinkle each serving with 1/2 teaspoon sesame seeds.
Wine note: When layering potent flavors like the savory soy and char-grilled smoke of Grilled Skirt Steak and Mesclun Salad with Miso Dressing, the wine should have similar complexity. A supple syrah, like Gloria Ferrer Syrah 2002 ($19), resonates with this dish: It has just enough tannin to stand up to the steak, and the ripe blackberry and smoky flavors are amplified by the savory elements. --Jeffery Lindenmuth