In a food processor, combine all-purpose flour, ¼ cup grated Parmesan cheese, Italian seasoning, freshly ground black pepper, sugar, and kosher salt. Pulse until mixed, then cut in very cold, diced unsalted butter until the mixture resembles coarse crumbs. Gradually add ice water, pulsing until a dough forms. Shape into a disk, wrap in plastic, and chill in the refrigerator for 1 hour to firm up.

In a large skillet, heat olive oil over medium heat. Sauté thinly sliced onions until they become translucent, about 5 minutes, stirring frequently to prevent burning. Add minced garlic, cooking for another 1-2 minutes until fragrant and soft. Remove the mixture from heat and allow it to cool slightly.

Preheat your oven to 400°F (200°C) to prepare for baking. On a lightly floured surface or parchment paper, roll out the chilled dough into a 12-inch diameter circle. Transfer the dough to a baking sheet, then layer on the cooled sautéed onions, shredded cheese, fresh herbs, and sliced tomatoes, leaving a 2-inch border.

Season the tomatoes with additional kosher salt and freshly ground black pepper to taste. Carefully fold the edges of the dough over the filling, creating a rustic border.

Brush the exposed edges with heavy cream. Place the assembled galette into the preheated oven and bake for about 45 minutes, or until the crust is golden brown and tomatoes are tender.

Let the Tomato Galette cool for about 10 minutes. Serve warm as a stunning appetizer or a light meal.