Preheat the oven's broiler and set the oven rack about 6 inches from the heat source. Arrange bread slices on a baking sheet in a single layer.
Broil bread slices until toasted, 1 to 2 minutes; watch carefully to prevent burning. Remove toasted slices and set aside on baking sheet.
Heat olive oil in a skillet over medium heat; cook and stir mushrooms, green onions, garlic, salt, black pepper, and parsley until mushrooms are softened and juicy, about 5 minutes. Set mixture aside. Mix diced tomatoes with balsamic vinegar in a bowl and set aside.
Top each toasted bread slice with mushroom mixture and spread tomato mixture on top. Spread mozzarella cheese over each appetizer. Return to broiler until cheese has melted and bruschetta are hot, about 5 minutes.
