In small saucepan whisk together balsamic vinegar and brown sugar. Bring to low boil and reduce by half stirring several times. Remove from heat and allow to cool.
In bowl combine diced tomatoes (both fresh and sun-dried), minced garlic, 1 teaspoon olive oil, fresh basil and salt & pepper to taste.
Brush mushrooms with olive oil and broil for 8 minutes turning halfway through. Top with shredded mozzarella and place back under broiler for 1-2 minutes or just until the cheese begins to brown.
Remove from oven divide bruschetta evenly placing on top of mushrooms. Drizzle with cooled balsamic reduction sauce and serve.