(Can this(+) etymology be sourced?) For the first part, compare the older ‘ghewendt broot’ (variants: ‘gewonden brood’, ‘gewonnen brood’) which meant ‘bread soaked in eggs, confectionery bread’. The meaning could be related to soaking, to turning around (of either the egg mixture or the bread) or even to winning something back (they're traditionally made from stale bread; compare French ‘pain perdu’).
The origin of the second part is completely unknown, but is variously theorised to have got something to do with baking, confectionery, or a slice of bread. It is possible (but not known) that the form of this part changed under influence of the word ‘teef’.
There are also various folk etymologies, like ‘wentel 't even’ (just flip it) or ‘wentel’ (flip) + ‘teef’ (bitch), but these are unlikely when the older terms are taken into consideration. The interpretation of the second part as ‘bitch’ did influence the Frisian word for French toast: ‘brette hânsfuotsjes’.
wentelteefje n (plural wentelteefjes)
It is more common to use the plural form; the singular form is only used when referring to one specific piece.