Preheat broiler. Remove and discard stems from mushrooms; with a spoon, scrape and remove gills. Brush caps with 4 teaspoons oil. Place in an ungreased 15x10x1-in. baking pan, stem side down. Broil 4 in. from heat 5 minutes.
In a small bowl, mix tomatoes, cheese, Italian seasoning and remaining oil. Remove mushrooms from broiler; turn over and fill caps with tomato mixture.
Broil 4-6 minutes longer or until mushrooms are tender and cheese is melted. Serve on buns.
Editor's Note: Mushrooms may also be grilled. Place mushroom caps on grill rack over medium heat, stem side down. Grill, covered, 5 minutes. Remove from grill; fill caps with tomato mixture. Grill, covered, 4-6 minutes longer or until mushrooms are tender and cheese is melted. Serve on buns.