In skillet over medium-high heat, add butter, olive oil, onions and salt and cook about 20 minutes. Add balsamic vinegar, honey and thyme and lower heat to medium-low. Cook, stirring continually, until onions are cooked evenly and caramelized, about 30 minutes. (Note: The onion will turn translucent and with an amber-golden brown color.)

Deglaze onion from pan with 2 tablespoons water to prevent sticking upon removal from skillet. Set aside to cool for assembly.

Preheat oven to 400 degrees F.

On baking sheet, unroll 2 defrosted pastry dough sheets and, using ravioli cutter or sharp knife, cut into 12 3-inch squares.

With sharp knife, carefully score borders of squares. (Note: Scoring edges will help border rise when baked and create a ledge to contain filling.)

In small bowl, crack and mix egg. Using pastry brush, brush egg-wash on edges of dough. (Note: Egg wash creates nice color on tart.)

Bake 15-18 minutes, until dough puffs up and turns golden color.

To serve, if desired, drizzle honey on top and sprinkle on more herbs.