Q. What is HoRav Shlomo Miller’s opinion on quinoa products on Pesach? Is quinoa cooking oil permitted? If you consider quinoa to be kitnios, can you use plates that were used to serve them when they are still ben yoma, like in a hotel?

A. Quinoa, is a grain-like seed grown in the Andean mountain region of South America, that is gaining popularity with consumers because its unique nutritional qualities.
Poskim disagree if quinoa seeds should be treated like potatoes and dissimilar to kitniyos, or maybe it is more alike peanuts and should be prohibited for Ashkenazim during Pesach.
Several Kashrus agencies, including the Star-K, OU, cRc (Chicago) as well as the Kof-K maintain that quinoa is essentially Kosher for Pesach. Since it is not even remotely related to the five grains (in fact, it is also not a legume and not botanically related to peas and beans which are of the original species of Kitniyos included in the decree), and was not around at the time of the Kitniyos prohibition, it is not considered Kitniyos. (ohr.edu)
However, other kashrus agencies disagree, such as OK, COR, Badatz of the Eida Chareidis of Yerushalayim..Additionally, Poskim as Rav Yosef Shalom Elyashiv zt”l, Rav Yehoshua Yeshaya Neuwirth zt”l, Rav Asher Weiss and the current Ashkenazi Chief Rabbi of Israel, Rav Dovid Lau maintain that it is prohibited. (ibid.)
Horav Shlomo Miller’s Shlit’a opinion is to be stringent in the use of quinoa on Pesach. In need you may use the cooking oil. However, you can be lenient in regards to utensils and plates on which quinoa products were served.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a.