Caramelized Onion Tart with Gorgonzola and Brie

Name: Caramelized Onion Tart with Gorgonzola and Brie
Ingredients: 2 tablespoons extra virgin olive oil; 4 cups sliced onions, sliced root-to-top into 1/4-inch thick slices (about 2 to 3 medium onions ); 1 tablespoon brown sugar; 2 tablespoons balsamic vinegar; 1/2 teaspoon salt; 1/2 teaspoon pepper; 8 ounces (1 sheet) frozen puff pastry, defrosted a couple hours in the fridge; 2 ounces chilled brie cheese , rind removed and diced; 2 ounces Gorgonzola or other blue cheese , diced; 2 tablespoons chopped fresh tarragon