Preheat the oven to 375 degrees Fahrenheit.
Cut the tops off the tomatoes and scoop out the flesh, being careful not to pierce the bottoms. Season the insides with salt.
Purée the tomato flesh in a small food processor. Set aside.
In a large skillet, sauté the onion over medium-high heat in 2 tablespoons of olive oil until soft, 4 minutes. Add the zucchini and garlic and cook a few minutes longer, until the zucchini is just beginning to brown, 3 minutes. Stir in the salt and rice, cooking until pearly white and toasty, 2 minutes. Carefully pour in the tomato mixture and add the tomato paste. Simmer until just thickened, 2 minutes. Remove from the heat.
Arrange the tomatoes in a large casserole dish. Spoon the rice mixture into the tomatoes until just shy of full (you might have a little extra depending on the size of your tomatoes). Replace the tops and season generously with salt and pepper. If using, stick the potato wedges in between the tomatoes. Drizzle them with olive oil and a pinch of sea salt. Pour 1 cup water around the tomatoes. Bake in the oven for 1-2 hours, until the tomatoes are caramelized and the rice is al dente. Serve hot or at room temperature, garnished with chopped parsley.