It's a play on words. Are you familiar with the British meaning of the word "blooming"? Check it out if not. And then of course there is the Blooming Onion which became a popular item at Outback Steakhouse restaurants. :)
There's a true story behind this meme. My husband took me on a romantic getaway weekend for an anniversary and gave me a Blooming Onion cutter. :D He got a good laugh from my reaction. Later he gave me a ring. :)
So glad you said "booming onion" because for some reason I googled "onion bomb" and found these. I had never heard of them before. They look delish! :)