In medium bowl, mix Italian dressing and 1/4 teaspoon of the basil. Add mushrooms, zucchini and bell pepper; toss to coat. Let stand at room temperature 15 minutes to marinate.
Meanwhile, heat closed contact grill 5 minutes. Sprinkle tuna steaks with seasoned salt and remaining 1/4 teaspoon basil.
When grill is heated, place vegetables on bottom grill surface; reserve dressing in bowl. Close grill; cook 3 to 5 minutes or until vegetables are crisp-tender. Remove from grill; return vegetables to bowl with dressing.
Add tuna to bottom grill surface. Close grill; cook 3 to 5 minutes or until fish flakes easily with fork. Serve vegetables and dressing with tuna.