In a saute pan over low heat, caramelize the onions with the butter. Cook over low heat until the onions are brown and soft.
Roll out the puff pastry onto a floured surface.
Put the puff pastry on a parchment lined sheet pan.
In a small bowl, combine the sour cream, mustard and salt and pepper to taste.
Spread the mustard mixture over the dough, fold the edges of the dough over to form a crust if you will.
Spread a layer of the onions over the mustard mixture. It acts like a barrier to prevent the crust from getting soggy.
Slice the tomatoes.
Arrange the tomatoes over the onion crust.
Season with salt and pepper..
Bake in a preheated 400 degree oven, about 25 minutes or until the puff pastry is brown and crisp. Have I mentioned that I lurve puff pastry
Get your feta and basil ready. Wait until the tart cools down a bit before you the herbs and cheese on.
Top with the feta and basil. You can serve this tart in small pieces for an appetizer or as a main course with a small salad. You can also play around with the cheese that you use, goat cheese would be great here too..