PREPARE tart shells as per package directions for unfilled tart shells. Remove from oven, set aside and cool.
MELT unsalted butter and oil in a large fry pan. Add onions to pan, and cook over medium-low heat until caramelized, soft and golden, about 20 minutes. Season with salt and pepper.
DIVIDE the softened goat cheese between the cooled tart shells. Top with the caramelized onion mixture and sprinkle fresh herbs over the top.