Cut the kernels off the corn cobs using a sharp knife. Toss with 1 tablespoon of the oil and a generous pinch of salt and pepper. Spread out on to a baking sheet and roast, stirring occasionally, until lightly browned, about 30 minutes.
Whisk the vinegar and mustard together in a large bowl. While whisking, slowing drizzle in the remaining 1/4 cup oil and with season with salt and pepper. Add the roasted corn, tomatoes, basil, scallions and beans and toss well.
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