Steven Universe is a show about, well, Steven Quartz Universe. The creator of this article the best at describing and he's lost some motivation for humor so please don't hate.
Steven is going everywhere in Beach City saying stuff about Cookie Cat until some gem monster pops up and acts like a mad man and does stuff. The gem monster picks up Steven and says in a creepy voice "HELLO THERE, MY DEAREST FRIEND. I'VE GOT SOME GOOD IDEAS FOR A LITTLE, CELEBRATION. I GOT SOME GOOD DEALS ON MERCURY FLAVORED... ..LASAGNA." but then the Space Moms appear and PWN the monster. It turned out to be Onion. Steven says "Oh guys, it's just innocent old Onion, stopping crimes that Onion did.". Onion is evil so he kills Steven, and Steven retreats to his gem.