8 slightly firm medium heirloom tomatoes
3 tablespoons vegetable oil, divided
3 cups white corn kernels (out of 3 to 4 cans)
2 cups chopped onions
2 garlic cloves, peeled
1 cup chopped tomatoes
3 cup ¼-inch cubes zucchini (from about 12 ounces)
2 tablespoons chopped cilantro
1 teaspoon dried oregano
Core whole tomatoes, creating 2 ½ -inch opening at top. Using melon baller, scoop out tomato, transferring juices and pulp to small bowl (for filling). Turn tomatoes, cut side down, onto paper towels to drain.
Heat 1 tablespoon oil in a large deep skillet over medium-high heat 1 to 2 minutes. Add corn and toss until tender and beginning to color, 2 to 3 minutes. Using slotted spoon, transfer corn to medium bowl. Add remaining 2 tablespoons oil to same skillet. Add onion and sauté until translucent, about 4 minutes. Using garlic press, squeeze in garlic; stir 30 seconds. Add chopped tomatoes and reserved tomato pulp and juices. Sauté until tomatoes are soft, about 5 minutes. Mix in zucchini, cilantro, oregano, and corn. Sauté until sauce thickens and zucchini is just tender, about 8 minutes. Season generously with salt and pepper.
Arrange tomato shells, cut side up, on small baking sheet. Spoon in filling, mounding high.
Preheat oven to 350°F. Bake tomatoes until filling is heated through and tomato shells are just tender, about 25 minutes.