I think I have come across the non-potato mash rather late in life, (but am now in the process of catching up) and am definitely a fan. For a very quick and simple mash, which involves no peeling, chopping or boiling, tinned cannellini beans are absolutely brilliant. You can add any of the flavours that you normally would to mashed potato such as garlic or herbs, or just keep it simple with butter and salt and pepper.
I like to gently sweat a clove or two of crushed garlic in a couple of table spoons of olive oil and once softened add in two tins of cannellini beans (makes enough for two hungry people). These will also start to soften as they warm through, and you can then fully squish them with a regular potato masher or stick blender. When smooth, return to a gentle heat to make sure they're piping hot and then add lots of salt and pepper and maybe a knob of butter. And that's it- a lovely, creamy mash that happily fills the place of mashed potato when you feel like a bit of a change or if time is of the essence.
NB The picture below does not really do it justice.
Tesco cannellini beans (410g)
I rate them 9/10
Cost: Around £0.45 depending on your supermarket