This is a juicy salad. Serve it on a platter lined with tender young greens, next to a grilled slice of olive-oil rubbed country bread or, best — over more ripe tomatoes. If you choose big, red beefsteaks for the salad, serve the salad over a different type of tomato, like slices of yellow tomato or a ring of halved pear or cherry tomatoes. With a small knife, cut the cores from the tomatoes and then cut the tomatoes into quarters through the core end. Cut the tomatoes into chunks about 1 1/2 inches. Toss the tomato chunks, salt, basil and red onion gently in a mixing bowl and let stand, tossing gently once or twice, from 10 to 15 minutes. Drain the tomatoes in a sieve over a small bowl and spoon off all but three to four tablespoons of the juice. (How much juice accumulates in the bottom of the bowl depends on the size and ripeness of your tomatoes. Save any leftover juice for another use: It is great in Bloody Marys, other salad dressings or stirred into tomato sauce.) Pour the olive oil slowly into the tomato juice, whisking constantly. Taste the dressing and add freshly ground pepper, more tomato juice or olive oil to taste. Pour the dressing over the tomatoes and taste for salt, adding more if necessary. Serve immediately.