Take a 2 1/2-inch round cookie cutter and cut out 12 rounds from 2 sheets of puff pastry. Take a 12 hole mini muffin pan and place rounds of dough into each hole, pressing gently to form a cup and giving room for the filling. Cover with plastic wrap and keep the prepared pan in the refrigerator or freezer (depends how long until you need it).
In a skillet add butter and oil. Throw in the onions and cook gently until softened and caramelized, stirring frequently for about 15 to 20 minutes. Once cooked add the scallions and chives. Set aside to cool slightly.
In a bowl whisk together the egg, cream, and salt. Add the cooked onions to the custard, stirring thoroughly. Pour into the prepared pastry cups with a spoon. Break up the goat cheese and place on top of tarts. Bake for 20 to 25 minutes. Serve.