Preheat oven to 425º
In a skillet, melt olive oil and butter over medium heat.
Slice onions.
Season onions with salt and fresh black pepper and add to the pan.
Stirring constantly, cook onions until they are soft and caramelized. Set aside.
Cut out six circles using a 4-inch biscuit or cookie cutter.
Place discs of dough on a cookie sheet lined with parchment paper.
Combine egg with cold water to make an egg wash.
Brush each disc with egg wash.
Place a slice of tomato on the center of each disc.
Add caramelized onions on top of tomato.
Add shredded cheese on top of onions.
Bake in preheated oven for 18 minutes or until cheese has melted and the tarts are golden brown.
Sprinkle parsley on top before serving.