In a saute pan over low heat, caramelize the onions with the butter. Cook over low heat until the onions are brown and soft, about 10 minutes.

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, 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 as 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.

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.