Any way you want, typically room temp. They are similar to smørrebrød in function just a completely different style of bread. smørrebrød is typically a weekday iem this is typically a weekend or holiday item.
You can use pretty much h anything to top, savory items get a layer of soft butter then meats, cheeses, shrimps, veggies etc. Same with smørrebrød but this being a weekend item typically gets the sweet treatment. Blackberry and fig are my two favorites, wife like lingonberry but damn expensive.
My family was here for the 4th and they brought something i can only find to import so frighteningly expensive and can melt along the way.... it is a chocolate wafer that after the butter you place on top and eat. Odd sounding I know but very common. Imagine a Hershey bar thin as a quarter, maybe a nickel. Good 'rememberies' of my frequent visits as a kid when mom was still alive.