Q) If I have eaten mezonos, and then I want to wash and have some bread, should I say al hamichyo in between?
A) As a general rule, the only reason not to say a brocho acharona after eating a “snack” before eating a meal is if the birkas hamozon after the meal will not cover the snack eaten before the meal. This will depend on the type of mezonos eaten prior to the meal:
• Regarding regular pas habo’oh b’kisnin, you should not say al hamichyo in between (Bi’ur Halocho 176 d.h. “borech”). The reason is as follows:
The definition of pas habo’oh b’kisnin is subject to major dispute among the Rishonim (see Shulchan Aruch OC 168:7). Therefore, even though the Shulchan Aruch (ibid) rules to say mezonos (and not hamotzi) on any item that is pas habo’oh b’kisnin according to any one of the opinions, before a meal it is not possible to make an al hamichyo until all opinions are satisfied that the item in question is not bread. This is because, as previously mentioned the only reason to say al hamichyo before a bread meal is if the food item that was eaten previously will not be covered with birkas hamozon at the end of the meal. Since the definition of pas habo’oh b’kisnin is subject to major dispute among the Rishonim, effectively each pas habo’oh b’kisnin item is actually a sofek hamotzi and may therefore be covered by birkas hamozon at the end of the meal.
• The only scenario that validates saying al hamichyo after eating pas habo’oh b’kisnin before starting a meal is if the food item in question is pas habo’oh b’kisnin according to all opinions (see Shulchan Aruch ibid). In such a case, all opinions will agree that the food item is mezonos, requires al hamichyo and will not be covered with birkas hamozon.
For example, if one eats a piece of apple pie which is baked with a filling, consists of a sweet dough and has a hard, brittle texture has eaten a food item that is pas habo’oh b’kisnin according to all opinions and should therefore say al hamichyo before starting a bread meal.
• A cooked mezonos food item (e.g. porridge) requires al hamichyo before starting a meal.