Right! That's how the tarts are removed from the pan, by pushing up on the little metal circle. Depending on the diameter, you might get away with using wide mouth canning jar lids, or even regular ones; it's hard to tell the diameter of the pan bottoms. Use them with the sealing compound side up and put a square of foil between the lid and the tart shell pastry. Or you could cut the bottom out of the foil pans from frozen pot pies or cut the circles out of larger metal pans like from under pizzas. The diameter of the piece of metal needs to cover the bottom of the tart pan so that it lifts the tart and does not just poke a hole through the bottom crust when you push up on it.