Reference for Oktzin 2:4
כָּל הַקְּלִפִּין מִטַּמְּאוֹת וּמְטַמְּאוֹת וּמִצְטָרְפוֹת. רַבִּי יְהוּדָה אוֹמֵר, שְׁלשָׁה קְלִפִּין בַּבָּצָל. הַפְּנִימִית, בֵּין שְׁלֵמָה בֵּין קְדוּרָה, מִצְטָרֶפֶת. הָאֶמְצָעִית, שְׁלֵמָה מִצְטָרֶפֶת, וּקְדוּרָה אֵינָהּ מִצְטָרֶפֶת. וְהַחִיצוֹנָה, בֵּין כָּךְ וּבֵין כָּךְ, טְהוֹרָה:
All peels can be rendered impure, and render impurity, and join together [with the food to count towards the measure for impurity]. Rabbi Yehuda says: the onion has three peels; the inner one joins together, whether it is whole or pierced; the middle one joins together when it is whole, and it does not join together when it is pierced; and the outer one is pure [i.e. does not join together, and is considered to serve neither as a handle nor to preserve the food] in either case.