How to Make It
Preheat oven to 350°F, with oven rack 6 to 9 inches from top. Cut about 1/2 inch off tops of tomatoes; discard tops. Cut a small (1/8-inch) slice off bottoms of tomatoes so they will sit flat in baking dish. Carefully scoop out tomato pulp, using a serrated tomato corer or melon baller, leaving a 1/2-inch shell intact. Sprinkle shells with 1/4 teaspoon of the salt. Place pulp in a wire-mesh strainer over a bowl; press to remove 1/3 cup tomato juice. Reserve juice; discard pulp.
Melt butter in a large skillet over medium-high. Add onion to skillet; cook, stirring occasionally, until starting to soften, about 4 minutes. Add spinach to skillet; cover and cook until wilted, 2 to 3 minutes. Add rice and reserved 1/3 cup tomato juice to skillet; cook until warmed through, about 1 minute. Remove from heat. Stir in basil, 1/3 cup of the pesto, and remaining 1/2 teaspoon salt. Divide mixture evenly among tomato shells. Place shells in an 11- x 7-inch baking dish.
Bake in preheated oven until tomatoes are tender, about 15 minutes. Top with cheese and remaining pesto.