If I ever open a restaurant, this will be the only thing on the menu. It is so good. I used to be a professional chef and worked at L'Auberge (a French restaurant in Portland) and at Genoa (a Northern Italian restaurant owned by the same person). We made a great many fantastic dishes at both restaurants, but nothing was as good as this.
First, boil and mash 3# potatoes with some olive oil. Add sliced onion that has been sauteed in olive oil, and some grated parmesan. I am lazy and mash potatoes in my stand mixer, but you have to be very careful not to over-do it and end up with gummy potatoes. And never, never use a cuisinart unless you need some spackle to fill holes in your walls.
Trader Joe's has these adorable little whole milk mozzarella balls, called ciliegine. I have no idea how to pronounce that, but it is the middle size of the three sizes available.
Cut the mozzarella in half and squish them down into the mashed potatoes.
Spread a bit of fresh tomato sauce (or in this case, leftover marinara) on top.
Sprinkle with parmesan cheese, sliced green and kalamata olives, and capers. Drizzle with olive oil and sprinkle with breadcrumbs. I like to hold a little cup of olive oil really high above the tart to get a thin stream and make a Jackson Pollack pattern ( unnecessary, but fun).
I got this recipe from a friend who lived in Eugene at the time. We used to cook together all the time. He took a cooking class from a woman who had just returned from a trip to Italy and this is one of the recipes she came back with. This recipe and one other from her (pepperonata) have been staples of my repertoire ever since.
Here is a copy of the original, oil spotted recipe.