Preheat oven to 375 degrees F. Line a large cookie sheet with parchment paper and spray paper with cooking-oil spray (or brush lightly with vegetable oil).
Have all your ingredients prepared in advance before preparing the Fillo Dough. This is called Mise en Place.
Preparing Fillo (Phyllo) Dough: When you open the package you will then unroll all the sheets on a flat surface. They sheet are very thin. Lay them on top of parchment paper so that they do not stick to anything and cover the top with a piece of parchment as well.
Lay one (1) sheet Fillo (Phyllo) dough on the prepared parchment paper on the cookie sheet. Brush lightly with a little melted butter. Do not worry if the dough tears a little while brushing with butter. As the phyllo layers are stacked, the only important one is the top layer.
Sprinkle all over with 1 tablespoon parmesan cheese. Repeat layering five (5) more times (with filo dough, butter, and parmesan cheese), pressing each sheet firmly so it sticks to sheet
Lay the last fillo sheet on top, brush with remaining melted butter, and sprinkle on remaining 1 tablespoon parmesan cheese.
Scatter onion slices across the top of dough, top with mozzarella cheese, and arrange tomato slices in a single layer (overlapping slightly). Sprinkle with thyme or basil and salt and pepper to taste.
Bake until the dough is golden brown, approximately 30 to 35 minutes. Remove from oven and let cool 10 minutes. Using your pizza cutter or a sharp knife and cut into approximately 3-inch squares.
Serves many as an appetizer - Makes about 20 (3-inch squares).
* Thaw frozen Fillo (Phyllo) dough in the refrigerator for at least 8 hours or overnight before preparing the recipe.
** Cut tomatoes very thin so juices evaporate while baking or dough will be soggy. Multi-colored Heirloom Tomatoes would also be beautiful on this tart.
Fillo Tomato Tart Recipe: https://whatscookingamerica.net/vegetables/filotomatotart.htm