GRILL DIRECTIONS: In jar with tight-fitting lid, combine all dressing ingredients; shake well. In small saucepan, cook potatoes in small amount of water for 15 to 25 minutes or until tender. Cool slightly; slice.
Meanwhile, wash and trim fresh beans; break into 1-inch lengths. In medium saucepan, bring 1/2 cup water to a boil. Add beans; cook 5 to 10 minutes or until crisp-tender. Drain; cool.
In nonmetal bowl, pour 1/2 cup of the dressing over warm sliced potatoes. Cover; set aside.
Place egg in small saucepan; cover with cold water. Bring to a boil. Reduce heat; simmer about 15 minutes. Immediately drain; run cold water over egg to stop cooking. Peel egg. Refrigerate until serving time.
Heat grill. When ready to grill, oil grill rack. Place tuna on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium coals. Cook 10 minutes per inch of thickness or until fish flakes easily with fork, turning once halfway through cooking.
To serve, line large platter with spinach leaves. Arrange marinated potatoes in a circle on spinach, 1 inch from outer edge of platter. Spoon cooked green beans into center. Flake grilled tuna; arrange over green beans.
In small saucepan, heat remaining dressing until warm; pour over salad. Mash egg with fork or press through sieve; sprinkle over tuna. Slice tomatoes lengthwise; arrange around edge of platter. Sprinkle salad with onions.