Defrost puff pastry according to package instructions and keep in the refrigerator. Remove from the fridge 15 minutes before use.
Peel and slice the onions into thin slices. In a pan, heat the herbs on low to release their fragrant oils. Take care not to burn them. Add butter and olive oil. Continuing on low heat, let butter and oil infuse with herbs - about one minute. Add onion, wine and vinegar. Cook over medium heat stirring occasionally until all liquid evaporates - about 30 minutes.
Meanwhile wash and cut the peppers into quarters lengthwise. Place them skin side up on a cookie sheet lined with parchment paper and bake in a preheated oven at 350 for about 15 minutes. Wrap the cooked peppers in foil and let steam for twenty minutes. Once cooled, peel the peppers and set aside.
Roll the puff pastry into a sheet large enough to cover a 9 inch non stick baking pan. Lightly oil the pan and arrange the pepper slices in a radial pattern. Sprinkle the peppers with a thin layer of grated gruyere cheese. Add the cooked onion and spread over the gruyere evenly. Sprinkle the chopped olives over the onion layer and top with another layer of gruyere taking care to cover the entire surface. Cover with puff pastry pressing the edges inside the pan. Bake in a preheated oven at 375 degrees until golden brown - about 35 minutes.
Pull the tart from the oven and let rest for 15 minutes. Invert a plate over pan and using pot holders to hold the plate and baking pan together, invert the tart onto the platter. Garnish with halved olives and few sprigs of fresh thyme. Serve warm..
Notes
Tart can be prepared and assembled up to one day in advance. Use any type of fruity balsamic vinegar you like. I used Sonoma Harvest fig balsamic vinegar.
Perfect for lunches and brunches, this tart can be served as a main course with a salad or as a side dish.
Recipe by Fraiche Pantry at https://www.fraichepantry.com/2015/03/28/bell-pepper-tart-tatin/