Caramelized Onions:

Melt the butter in a large saucepan.

Add the sliced onions.

Cook over low heat, tossing occasionally with a spatula, for 1 hour or until onions are soft, sweet, and golden brown.

Tomato Jam:

In a small saucepan, heat the olive oil on the stovetop over medium heat.

Add the tomato paste and ketchup and cook for 1-2 minutes, stirring occasionally with a heat-resistant spatula.

Stir in the chopped tomatoes, brown sugar, garlic powder, paprika, red pepper flakes, and salt.

Reduce heat and simmer until tomatoes partially break down, about 15 minutes.

Remove from heat and transfer to a bowl. Whisk in the lemon juice.

Set aside to cool.

Tart:

Preheat oven to 400° F / 205° C. Line a half-sheet rimmed baking tray with parchment.

Place sliced tomatoes on a paper towel-lined plate to remove some of the moisture.

Unroll puff pastry on prepared tray.

With a rubber spatula, spread tomato jam evenly over the surface of the pastry, allowing a 1-inch border around all edges.

Place sliced tomatoes on top of the jam.

Scatter with the caramelized onions.

Top with the blue cheese.

Add the halved cherry tomatoes.

Sprinkle with grated Parmesan and red pepper flakes. Season with salt and pepper.

Fold the pastry around the filling, using your fingertips to seal the edges.

With a fork, crimp the edges of the pastry.

Brush the pastry with the beaten egg.

Bake in preheated oven for 25 – 30 minutes, or until puff pastry is deeply golden brown.

Sprinkle with chopped herbs. Garnish with additional Parmesan, if desired.

Slice into squares. Serve hot.

Wrap leftover tomato tart in foil and store in the refrigerator for up to two days. Eat leftovers cold, or gently reheat in a 350° F oven for 10 minutes, or until warmed through.