Complex sentence: Lemon curd is different from a lemon filling or a custard, in that it contains a higher proportion of lemon juice and zest which gives it a more intense/sharper lemon flavor.
Split into simpler sentences: Lemon Curd is different from a lemon filling or a custard, in that it contains more lemon juice and zest than lemon filling or custard. This gives lemon curd a more intense/sharper lemon flavor.