Roll out your puff pastry, lay it onto a baking sheet with parchment paper, take a fork and gently poke your puff pastry all over. This will keep it from puffing up too much as you bake your tart. Leave in the fridge until needed.

Heat your oven to 400 degrees.

Cut the stem of the onion leaving the root intact. Place the onion with the cut stem side down on the cutting board. This will keep it stable. Cut the onion in half through the root end. Peel back the onion skin and place the onion half cut side down and slice straight down so you have half-moon slices.

Heat your olive oil and butter in a wide-bottomed pan over medium heat. Add your onions and thyme season with salt and cook for 45 minutes to an hour, stirring often, until the onions are golden. you want to be careful that the onions don’t burn so you may need to adjust your heat accordingly.

Once the onions are golden and delicious add your Madeira and deglaze the pan.

Remove your puff pastry from the fridge. Spoon the onions onto the center of the pastry, leaving 1/2 inch around the edge as a border. Brush the border with your egg wash and bake in the oven for 30 minutes until the pastry is cooked and golden. Remove from the oven and let cool for 5 minutes. Top with your herbs, drizzle with aged balsamic, and top with Maldon salt.