Forgot to eat Kazayit of bread



I was planning to eat a little of bread to make a seudah , less than a Beitzah I washed without a bracha, said Hamotzi and took a bite of the bread. I put the bread on the side of my bowl of pasta and forgot about it. I washed Mayim Acharonim and realized I forgot to eat a Kazayit of bread. Was I able to continue eating more bread to make myself obligated in Birkat Hamazon, or since I did Mayim Acharonim I can’t? What should I have done?



You must say Al Hamichya on the pasta and then eat the bread with a bracha.

The Gemara  in Brachot 45a writes; Immediately after washing (Mayim Acharonim) you must say the bracha.
Rashi explains :It is referring to Mayim Acharonim and Birkat Hamazon and one may not eat after washing until he says Birkat Hamazon. So writes the  Tosfot. The Rambam (Brachot 6:20) writes: Therefore do not separate between them (washing and Birkat Hamazon) even to drink water.
The Kesef Mishna understood from the Rambam that is forbidden to eat or drink between Mayim Acharonim and Birkat Hamazon, but one may speak. In the Bet Yosef however he writes that the Tur in the name of the Rosh that it is forbidden to even speak between the two.

There is a different disagreement about someone who wants to eat after Mayim Acharonim, is he allowed to eat it just he needs to say a bracha Rishona or he is completely forbidden and he must say Birkat Hamazon and then say a new bracha.

The Rosh writes (Brachot 6:31) :As long as you have not washed you may eat. It seems to say that if you washed it is forbidden. The Rabbeinu Yonah agrees even with a bracha it is  forbidden. The Bet Yosef explains (179) that washing Mayim Acharonim is the first part of the bracha since the Gemara says you must put them together. One  a person sets himself up to say bracha tis as fi he started already therefore he may not eat before he says Birkat Hamazon.
Tosfot though (Chullin 80b) and Rosh in Pesachim are of the opinion that after one washes Netillat Yadayim  one may eat or drink if he says a bracha Rishona. The Bet Yosef disagrees and in Shulchan Aruch (179:1) he rules: One who finished his seudah and washed Mayim Acharonim may not eat or drink until he says Birkat Hamazon.
The Mishna Berura quotes the argument and sys the most Poskim rule that you may say bracha Rishona . The Biur Halacha however writes that one shod be careful not to get caught up in the disagreement, one should say  Birkat Hamazon and then go back and say a new bracha.
In our case though , he isn’t obligated in Birkat Hamazon just Al Hamichya is he required to say Al Hamichya?

Some want to differentiate that the Shulchan Aruch who says one may not separate between washing and Birkat Hamazon is only because the washing Mayim Acharonim is the first part of the bracha and it is forbidden to eat in the middle of a bracha. If he is not obligated in Birkat Hamazon The Mayim Acharonim has no connection, since Netillah was by mistake . Therefore they say in this case one may just continue eating even without saying a bracha Acharona. However I don’t think the Mayim Acharonim was a mistake. Firstly Mayim Acharonim is for cleanliness, or else to wash off S’dom Salt from your hands. Even or any bracha Acharona your hands have to be clean to say it and you need to wash them. Therefore one must wash his hands and they might still be considered part of the bracha and it is forbidden to eat other things until you say Al Hamichya. Then make a new bracha.