The correct spelling for the phrase "most heather" is /məʊst ˈhɛðə/. The first word "most" is pronounced with a schwa sound for the vowel "o" and a clear "st" ending. The second word "heather" is pronounced with a clear "h" sound, followed by a short "e" and a soft "th" sound, and finally, a clear "er" ending. This phrase may be used to describe something that is primarily composed of or covered in heather.