Is there any Torah source or basis for the minhag not to eat nuts on Rosh Hashana?
Is it really preferable not to?
What if the nuts are ground into a flour and not recognizable as nuts…?
Yes, this minhag is quoted by the Rama 583:2. 2 reasons are given:
1) In hebrew they have the same numerical value as cheit – sin.
2) They cause increased saliva which disturbs one’s davening.
It is generally assumed that ground nuts mixed in to another recipe are not included in this custom.