Drain the can of sardines well. Rinse the white beans under cold running water and let them drain as well in a colander. Meanwhile, finely mince the quarter red onion and chop the fresh parsley.
In a small bowl, mix the lemon juice with the extra virgin olive oil to make the dressing. Season with Celtic sea salt and pepper. Then take a large salad bowl and place the arugula, drained white beans and finely minced red onion in it.
Break the drained sardines into coarse pieces with a fork and add them to the salad bowl. Pour the dressing over everything. Gently toss the salad so the ingredients mix well but the sardine pieces remain whole.
Finally, sprinkle the finely chopped fresh parsley over the salad. Serve the dish immediately on plates.