How would you exaggeratedly express food that tastes nothing to sound even worse than "It tastes like cardboard"? I want a word that sounds beyond a average man can imagine. "Shit" would have its unique flavour, so I wouldn't put that in the list.
Shoe leather (I'm thinking of Charlie Chaplin trying to eat his boot).
Stale matzoh. (My mother described the most successful diet she ever went on like this: she stopped taking a lunch to work. The only food in her office was a box of ancient matzoh.)
Banal is a rather harsh adjective you could use to describe tastelessness.
If you wish to really exaggerate you could say: "The dish was mortally banal"
Food that's particularly bland and tasteless is often described as pablum (or Pablum):
a brand of soft, bland cereal for infants
It's amusing to note that when Pablum was introduced in 1931,
media reports at the time said the cereal tasted like "boiled Kleenex" and "had the consistency of mucilage and smelled like the inside of an old cardboard box."
shirt backing I once had an omelet in England that tasted like shirt backing. (same as cardboard but sounds cooler).