Start by caramelizing the onions: heat the onion slices with some salt and pepper in a non-stick skillet for about 30 minutes, stirring occasionally, until they are soft and golden brown
While the onions are cooking, grate the 3 cheeses and cut up the basil leaves.
Spread the puff pastries onto a greased baking sheet, gently stretching it to fill the whole sheet, you may need to use part of a second pastry.
Layer on all of the ingredients except for the basil. Starting with the cheeses, then the onions, and finally the tomatoes.
Bake the tart for 15-18 minutes, checking on it occasionally for bubbles in the crust. If bubbles appear, just stab them with a fork or knife.
When the crust is golden brown and the tomatoes look wrinkly, pull it out, allow to cool for a few minutes and sprinkle on the basil.
