2 tablespoons extra-virgin olive oil
1 teaspoon cider vinegar
1 1/4 pounds tomatoes, cut into bite-size pieces
1 bunch scallions, finely chopped, keeping white parts and greens separate
2 tablespoons unsalted butter
4 cups corn kernels (from about 8 ears)

Whisk together oil, vinegar, 3/4 teaspoon salt, and 1/4 teaspoon pepper. Toss tomatoes with dressing.
While tomatoes marinate, cook white parts of scallions in butter with 3/4 teaspoon salt and 1/2 teaspoon pepper in a 12-inch heavy skillet over medium-high heat, stirring occasionally, until golden, about 4 minutes. Add corn and sauté until just tender, about 5 minutes. Transfer to a bowl and cool.
Stir together corn, tomatoes, and scallion greens.