Sprinkle the cut sides of the tomato halves with salt, and place cut sides down on paper towels for 15 minutes to drain. Place frozen spinach, water, and chicken bouillon granules in a saucepan over medium heat; bring to a boil, cover, reduce heat, and simmer until spinach is tender, 8 to 12 minutes. Drain the spinach well. Preheat oven to 350 degrees F (175 degrees C). Place tomatoes, cut sides up, into a 9x12-inch baking dish. Mix together the spinach, 1/3 cup of Parmesan cheese, cornbread stuffing mix, melted butter, egg, onion, garlic, and black pepper in a bowl until thoroughly combined. Mound the mixture on the cut tomato halves. Bake in the preheated oven until cooked through and hot, about 15 minutes. Sprinkle each tomato with about 1 teaspoon of Parmesan cheese, return to oven, and bake until the topping has browned, about 5 more minutes. Serve immediately.