No king ever knighted a piece of beef. The "sir" in sirloin is a spelling accident that a good story later moved into.
The word is French. Etymonline traces it to surlonge, made of sur, "over, above," and longe, "loin." The French word meant "upper part of the loin." English had it by the early 15th century as surloine, with variants like surloyne and shurleyne. It first named a cut of beef in general, and from the 16th century that upper part specifically.
Longe itself came from Old French loigne, "hip, haunch." That goes back through a Vulgar Latin phrase meaning "meat of the loin" to Latin lumbus, "loin."
The sir- spelling shows up in the 1620s. Etymonline suggests the word sir simply pulled it over. Early modern English writers did the same thing to surname, which they often spelled sirname.
Once the spelling looked like a title, someone needed a knight. The earliest version Etymonline cites is in Thomas Fuller's Church-History of Britain of 1655. There, Henry VIII is the king who dubbed the beef. Later tellers handed the honor to James I or Charles II. The early 19th-century lexicographer Charles Richardson is said to have credited James I.
The James I version has a real setting. The king stayed at Hoghton Tower in Lancashire from 15 to 18 August 1617, and the menu of his feast survives. Wikipedia's page on the house says that menu is the source of the "erroneous" claim. It adds that there is no reliable evidence for the knighting and that scholars generally hold it to be a myth. A nearby pub was named after the supposed event anyway.
The dates also sink the story. In 1882 the etymologist Walter Skeat pointed out that the word was in use before James I was even born. He called the tale "one of those unscrupulous inventions." Etymonline sums it up plainly: there is no evidence for it.
So the knight is fiction, but the spelling he inspired is real. A cut named for its position, "above the loin," now carries a title it never earned.