Combine all the ingredients in a medium jar with a lid, seal and shake till well combined. Alternatively, you can whisk them all in bowl and then transfer to a container and store in the fridge till needed.
If you make this well in advance, you will need to bring it to room temperature before using.
Instead of white cannellini beans, feel free to use chickpeas, black beans, brown rice, quinoa, or even cooked lentils.
All ingredients for the salad are approximate, feel free to use more of the ones you like, etc. You can use larger cans of beans or tuna as well.
Use Castelvetrano olives if you want to take this more Italian.
Pickled Red Onion recipe is here.