Cut about 1 inch off the top of each tomato. Use a spoon to scoop out the insides, leaving 1/4-1/2 inch shell, being careful not to cut into the outside of the tomatoes. Discard the pulp. Sprinkle the insides of the tomatoes with the salt and place upside down on a double layer of paper towels to drain.
In a small bowl, combine 1 tablespoon of the oil, 1 tablespoon of the parsley, half of the garlic and the breadcrumbs. Stir until the breadcrumbs are coated and it is all mixed thoroughly.
Heat the remaining 2 tablespoons of oil in a large sauté pan over medium-high heat. Add the zucchini and onion and cook until translucent, 3-5 minutes. Add in the spinach and cook until it is wilted, about 1 minute longer. Stir in the remaining 2 tablespoons of parsley and the remaining garlic. Season with the oregano and the pepper. Remove from the heat and pour into a bowl. Let it sit for about 5 minutes, stirring occasionally, to slightly cool.
Add the cubes of cheese and the olives to the zucchini mixture and stir to combine.
Place the tomatoes, open side up, in a small baking dish. Divide the cheese and zucchini mixture between the tomatoes. Top with the breadcrumb mixture.
Bake in the preheated oven until the tomatoes are soft and the topping is golden, about 30 minutes. Serve warm.