There are two ways to serve this colorful steak-and-veggie salad: simply pile it on a bed of lettuce and dig in, or turn it into sandwiches by stuffing it into pita bread halves.
Heat the grill at the medium-high setting. Sprinkle the steak with salt and pepper to taste. Grill the meat (about 7 to 9 minutes per side on a gas grill for medium rare, longer if you prefer medium or well done). Let the steak cool, then cut it in half lengthwise and slice each half across the grain into thin strips.
Meanwhile, brush the yellow squash and zucchini with 2 tablespoons of the lemon juice and season it with salt and pepper. Grill the sliced vegetables, turning them once, until tender (about 2 to 3 minutes). Then cut them into thirds.
In a food processor, combine the parsley, basil, garlic, Dijon, and remaining tablespoon of lemon juice and blend well. With the motor running, slowly add the oil and blend well. Season with salt and pepper to taste.
In a large bowl, combine the steak, squash, zucchini, and onion, if desired. Add the dressing and toss to mix. Sprinkle with more salt and pepper, if desired. Stir in the tomatoes and serve. Makes 6 servings.