Ashkenazim have the custom not to eat "Kitniyot"-various beans and legumes on Pesah. The earliest authority to record this custom is the SaMaK, (Sefer Misvot Katan by Rabbi Yishak of Corbeil, 13th century) who lived 750 years ago. The reason he gives is that these foods can be confused with Hames grains, such as wheat. Just as wheat can be ground into flour, so too corn and lentils can also be dried and made into flour. Interestingly, he also includes mustard seed in the list of "Kitniyot." Even though it is not made into flour, nevertheless, it grows in pods and could also be mistaken for wheat.
In later generations, the Ashkenazi Poskim discuss whether other foods, such as coffee beans, peanuts and quinoa, would also be considered "Kitniyot." Rav Shlomo Amar and Rav Moshe Feinstein rule that any food that wasn’t known in the Western world at the time of the original institution of the Gezera was not included in the custom.
There was a controversy with regard to whether potatoes are considered "Kitniyot." Some held that since they were common at the time of the Gezera, and they can also be used to produce starch, they should be included in the custom. Nevertheless, the predominant custom is for Ashkenazim to use potatoes on Pesah.
These Halachot are relevant for a Sepharadi who hosts an Ashkenazi on Pesah. One should only serve an Ashkenazi guest foods that are not Kitniyot. However, there is no problem using the same pots used to prepare Kitniyot. The Gezera did not extend to the taste emitted from the pots.
SUMMARY
An Ashkenazi guest should only be served non-Kitniyot foods on Pesah.
