"The boy's sandwich"
We may never know.
Ways of forming the possessive in Norwegian:
"guttens smørbrød" (genitive -s added as a suffix; neutral, suitable for all occasions.)
"smørbrødet til gutten" (prepositional structure akin to "the sandwich of the boy" in English; less used than the above, but mostly on account of being more verbose. The extent of the usage is somewhat dialect dependent.)
"gutten sitt smørbrød" (possessive pronoun; this structure is called "garpegenitiv", and is not suitable for formal writing. Sees more frequent use in connection with proper names than with regular nouns.)