We figured out the thing we wanted to see in the world, so over some BBQ we outlined the vision, MVP and timescales for what we wanted to achieve. We had the YC video about how to build an MVP in mind, and decided on a date to launch, and what minimum really means in mind, and started to build to the barest version of what our vision was.
We wrote more about it on our blog https://taskablehq.com/blog/food-product-market-fit-manifesto