Preheat your oven to 425 degrees F. If you’re using frozen puff pastry, make sure to follow the package instructions to thaw it.

In a skillet, heat 1 tablespoon of olive oil over medium heat. Add the sliced fennel and red onion, cooking for about 10 minutes. Season with a pinch of salt and 1 tablespoon of sugar, then continue cooking for another 5 minutes. Add 2 tablespoons of water and scrape any brown bits from the bottom. Cook until the water evaporates, then transfer the vegetables to a paper towel-lined plate to drain.

In an oven-safe skillet, combine 1/4 cup butter and 1/4 cup balsamic vinegar over medium-high heat. Once the butter melts, add 1/4 cup sugar and stir well. Bring this mixture to a strong simmer, adding in 1-2 sprigs of thyme. Stir constantly for about 4-5 minutes until a caramel forms.

Remove the skillet from the heat and carefully arrange the halved tomatoes cut side up in a circular pattern at the bottom of the skillet. Season with salt and pepper, and sprinkle on some thyme leaves. Top the tomatoes with the sautéed fennel and onions, then cover with the puff pastry, tucking the edges into the pan.

Place your skillet in the preheated oven and bake for about 30 minutes, or until the puff pastry turns golden brown. Allow the pan to cool for approximately 5 minutes.

Using a butter knife, gently release the edges of the pastry from the pan. Place a serving plate over the skillet and carefully turn it over to release the tart. If any tomatoes remain in the pan, place them back on the tart. Garnish with fresh thyme leaves.