This is me trying to be helpful, it was covered in know your meme pretty well here: https://knowyourmeme.com/memes/avocado-toast
The term became a popular reference and in-joke for millennials after an article about millionaire Tim Gurner featured a quote from him saying he got rich by saving and not spending money on frivolous things such as avocado toast.
Edit: I don't think Gurner is a boomer judging by his pic. I can't find an age on him.