While the onion cooks, use a serrated-edge knife to shear the corn kernels off the cobs into a bowl to catch them. Add the corn to the onion and saute for a minute or two until the corn is just tender. Add the tomatoes and arugula and give it a stir just until the arugula wilts. Add the 1/4 teaspoon of salt, stir, taste, and add another 1/4 teaspoon salt, if desired. Transfer the corn salad to a serving platter.