Empty the cans of beans into a colander and rinse them briefly under cold water. Drain thoroughly and empty the colander into a large serving bowl. Toss the beans together with the red onion and arugula.
Shake the olive oil, vinegar, chopped basil, salt and pepper in a sealable container until the salt is dissolved. Pour the dressing over the salad and toss well. It's best to make and dress the salad about an hour before you serve it. Let it stand at room temperature, tossing every time you think about it. Just before serving, adjust the seasoning with salt and pepper, to taste.