Place pie shells on a baking pan. On your work space, arrange the ingredients for the pies: sliced onion, sliced tomato and cheese mixture. (It’s helpful to divide the cheese mixture in half so that you get an equal amount in each shell.) Place a few pieces of sliced onion in the bottom of each shell. Spread a thin layer of cheese mixture. Top with a slice of tomato (save the largest slices for the next layer), then a few more slivers of onion. Top with another thin layer of cheese mixture. Repeat with tomato, onion, then one last layer of cheese mixture. The filling should be higher than the edge of the pie shell; don’t worry, it will all melt and cook down. You should have a good amount of sliced onion and some tomato pieces left when your pies are fully assembled.