Preheat oven to 375ºF with a rack in the center position.

Slice about ½ inch off the top of each tomato and set aside. Cut out and discard the tomato cores; scoop out the pulp and juice, adding enough water or tomato juice to make about 1 cup, and set aside.

Combine sugar if using with 1 tablespoon kosher salt (2 teaspoons table salt) and sprinkle inside each tomato. Turn tomatoes upside down on a rack set over clean towels or a baking sheet and set aside to drain.

Heat 2 tablespoons olive oil in a skillet over medium heat; add the green onion bottoms and garlic and cook, stirring occasionally, until onions are very soft, about 7 minutes. Do not brown. Add pepper flakes to taste then stir in spinach until wilted. Stir in reserved tomato juice, rice, Cheddar, lemon zest, and lemon juice; taste and adjust seasoning as desired.

Spread remaining olive oil in the bottom of a baking dish large enough to accommodate the tomatoes. Wipe out the tomatoes with a clean towel and season tomatoes and tops with salt and pepper. Arrange tomatoes in prepared baking dish, divide rice mixture among the tomatoes, top with seasoned tops, and bake, uncovered, about 20 minutes. Carefully transfer to a serving platter. Whisk lemon juice into pan drippings and drizzle over each tomato; serve hot.