Preheat oven to 400°. In a large skillet, heat oil over medium-high heat; saute mushrooms and onions until tender. Add garlic and seasonings; cook and stir 1 minute.
Place baguette halves on a baking sheet, cut side up; sprinkle with half the cheese and ½ cup basil. Top with mushroom mixture, tomatoes and remaining cheese.
Bake until cheese is melted, 10-15 minutes. Sprinkle with remaining basil. Cut each half into 3 portions.
