Prepare your short-crust pastry, press it into an 11-inch quiche or tart pan, and prebake (blind bake) according to the recipe's .
Set the golden crust aside.
To create the rich tomato sauce, coarsely chop the prepared tomatoes, making sure to reserve all their juices.
Heat the olive oil in a large skillet over medium heat and sauté the minced garlic for 1-2 minutes until fragrant.
Add the tomatoes, their juice, ½ teaspoon of the salt, parsley, basil, and a few grinds of black pepper.
Bring to a simmer and cook, stirring occasionally, until the sauce has reduced by about half and is wonderfully thick. This can take 20-30 minutes.
While the sauce simmers, peel, halve, and thickly slice the onions.
In a separate pan, melt the butter over medium heat and sauté the onions until they are soft and golden brown.
Sprinkle them with the remaining ¼ teaspoon of salt.
Now, let's assemble the tart.
Sprinkle the grated Parmesan cheese evenly over the bottom of the prebaked pastry shell.
Arrange the sweet, sautéed onion slices in a single layer over the cheese. Gently spread the thick tomato sauce over the onions, covering them completely.
Cut the mozzarella into thin strips or small cubes and arrange them evenly over the tomato sauce.
Finally, slice the pitted olives and scatter them over the top of the cheese.
Bake in a preheated oven at 375°F / 190°C for 35 minutes, or until the cheese is melted, bubbly, and lightly golden.
Let it rest for a few minutes before slicing and serving hot.
