Put the potatoes and bay leaf in a pan of water a, bring to the boil, then cook for about 15 minutes until tender. Drain, then rinse under cold water. Drain again, then cut the potatoes into bite-sized chunks, if needed. Put them in a mixing bowl and season with salt and pepper.
Meanwhile, mix the mayo with mustard, vinegar and oil.
Drain the tuna, then add it to the dressing.
Cut the onion into tiny bits, add these to the salad. You can add a pinch of cayenne or chilli flakes too.
Scatter the olives over the potatoes, then gently fold in the tuna mixture. Season with some salt & pepper, then taste it and adjust the vinegar, mustard, mayo etc to your liking.
Scatter some chopped parsley on the top and serve.