Also, you'll need small tart pans or ramekins. Ours are about 3.5 inches in diameter. You use what ever you have. Small bowls. Cups. Whatever. Just make sure they're ceramic or metal. Not plastic, right? Right.
Olive oil in a pan over medium heat. Add onions and immediately decrease the heat to low and stir. Do not let these brown. Cook them for an hour. I'm not kidding. Just stir them occasionally. Do not hurry them. You are missing the point if you turn up the heat to medium. Leave it low and have a drink. Or five. About half-way through, remove the puff pastry so it can defrost some. When the onions are caramelized, turn off the heat and add the mustard and chopped basil. Add 1/2 t of salt and some pepper. Taste. We needed more salt.
Flour a work surface and roll out the puff pastry to about 1/8 inch thick. Get your tart pan/ramekin and a sharp knife. Place the pan on the puff pastry and cut around it with the knife. It will feel slightly like a second grade craft project. Enjoy the memories. Put the pastry cutouts on a plate and put it in the fridge. Keep it cold or it won't puff.
Heat the oven to 375 F.
Take some honey and give the bottoms of the pan a light coating. It's ok if you get it on the sides. Whatever.
Cover the bottom of the pans with some tomatoes, skin side up. Then layer on some onion mixture. Then slap on a puff pastry top. Brush some egg on there. Sprinkle with salt. Bake until brown, between 10-30 minutes. I know that's a big spread, but we made these twice and each time took a different amount of time. Pull them out and let them cool for 10 minutes (no longer or the honey starts to set).
Heat up the oil in a small pan until a dipped basil leaf sizzles. Place the remaining leaves in the oil until crisp. Maybe a minute or less. Drain on a paper towel.
Run a knife around the inside of the pans. Put a plate on top of the pan, and using a hot pad, hold the pan against the plate and flip them over together.
Garnish the tomato top with a dollop of mascarpone. Stick a bail leaf on there in some artsy way that makes you feel good.
Eat.