The Plabs Burger knocked Old Man Jenkins out because it was poisonous (the newspaper says, "Plabs Patty KO's Old Man Jenkins"). However, he is seen smiling before falling over after eating the burger, showing that it may also have a good taste. One of the secret ingredients was a pinch of chum.
The Plabs Burger looked pretty much the same as a Krabby Patty. It has the bun, the greens, and everything else. The ingredients are seaweed, squid ink, a pinch of chum, and others.
The Plabs Burger's name is a combination of Plankton and Mr. Krabs' recipe, the creators of the burger.