In a high-sided saute pan with a lid, heat the olive oil over medium heat. Add the onions, season with the salt, cover and cook until they are wilted and soft, about 10 minutes.
Remove the lid and cook the onions until they are golden and caramelized, stirring often, about 20 minutes more. Stir in the garlic and saute 1 to 2 minutes. Add the tomatoes and cook an additional 10 minutes. Add the remaining ingredients, bring the mixture to a boil and then reduce the heat to a simmer. Stirring consistently, cook until the tomatoes breakdown and the jam becomes thick, about 20 minutes. Yield: 3 cups;.
Preheat the oven to 400 degrees F.
Roll the puff pastry out on a floured surface to a rectangular shape, about 12 by 10 1/2 inches. Transfer the pastry sheet to a parchment-lined baking sheet.
With the tines of a fork, dock a 1/2-inch border around the edge of the sheet and dock the center of the sheet a few times as well. This will prevent the dough from rising as it cooks.
Spread the Caramelized Onion-Tomato Jam evenly over the pastry, leaving a small border around the edge of the pastry. Layer the tomatoes, eggplant, yellow squash and zucchini in overlapping rows, or in whatever pattern you like. Sprinkle the roasted red peppers around the tart. Drizzle the assembled tart with a little olive oil, making sure to brush a little on the edges, season with salt and pepper and top with half of the fresh oregano.
Bake until the pastry is golden brown and puffed, 20 to 25 minutes. Remove from the oven and immediately sprinkle with the goat cheese and the remaining 1 1/2 teaspoons oregano before serving.