1. Adjust oven rack to middle position and heat oven to 375°F. Cut top 1/2 inch off stem end of tomatoes and set aside. Using melon baller, scoop out tomato pulp and transfer to fine-mesh strainer set over bowl. Press on pulp with wooden spoon to extract juice; set aside juice and discard pulp. (You should have about 2/3 cup tomato juice; if not, add water as needed to equal 2/3 cup.)

2. Combine sugar and 1 tablespoon salt in bowl. Sprinkle each tomato cavity with 1 teaspoon sugar mixture, then turn tomatoes upside down on plate to drain for 30 minutes.

3. Combine 1 1/2 teaspoons oil and panko in 10-inch skillet and toast over medium-high heat, stirring frequently, until golden brown, about 3 minutes. Transfer to bowl and let cool for 10 minutes. Stir in 1/4 cup mozzarella.

4. Heat 2 tablespoons oil in now-empty skillet over medium heat until shimmering. Add onion and 1/2 teaspoon salt and cook until softened, 5 to 7 minutes. Stir in garlic and pepper flakes and cook until fragrant, about 30 seconds. Add spinach, 1 handful at a time, and cook until wilted, about 3 minutes. Stir in couscous, lemon zest, and reserved tomato juice. Cover, remove from heat, and let sit until couscous has absorbed liquid, about 7 minutes. Transfer couscous mixture to bowl and stir in capers, pine nuts, and remaining 1/2 cup mozzarella. Season with salt and pepper to taste.

5. Coat bottom of 13 x 9-inch baking dish with remaining 2 tablespoons oil. Blot tomato cavities dry with paper towels and season with salt and pepper. Pack each tomato with couscous mixture, about 1/2 cup per tomato, mounding excess. Top stuffed tomatoes with 1 heaping tablespoon panko mixture. Place tomatoes in prepared dish. Season reserved tops with salt and pepper and place in empty spaces in dish.

6. Bake, uncovered, until tomatoes have softened but still hold their shape, about 20 minutes. Using slotted spatula, transfer to serving platter. Whisk vinegar into oil remaining in dish, then drizzle over tomatoes. Place tops on tomatoes and serve.

Serves 6