Flavonoid changes in sliced and fried onions which were packed and stored at different atmospheric conditions (air, nitrogen and vacuum), temperatures (ambient, +5 and −18C) and light (dark or light) were investigated. Flavonoids were extracted using accelerated solvent extraction and analyzed using ultra performance liquid chromatography coupled with photodiode array detector. Total flavonoid content, quercetin-3,4′-O-diglucoside and quercetin-4′-Omonoglucoside contents in sliced reference onion samples were found as 1,570 ± 176, 926 ± 105 and 564 ± 64 μg q.e./g d.w., respectively. Frying did not result in significant losses of flavonoids. At room temperature, total flavonoid losses were significant, besides conversion of quercetin glycosides into aglycons. Dark conditions better retained flavonoids of sliced onions at all atmospheric conditions. For sliced onions; +5C, air or vacuum atmosphere, or −18C, vacuum or nitrogen atmosphere, under dark, preserved flavonoids for 21 days, whereas for fried onions, 7 days of storage at +5C, vacuum atmosphere under dark resulted in highest flavonoid content. PRACTICAL APPLICATIONSOnion is one of the most widely consumed vegetable all over the world and also used in industrially processed forms for domestic and catering purposes. Onion includes high contents of flavonoids and has been attracting extra attention by consumers. Onion products are also preferred by consumers, but processing may be resulting in losses in those valuable flavonoids. Previous researches focused on the effect of domestic processing techniques such as chopping, shredding, peeling, roasting, cooking or boiling on flavonoid content and depending on the severity of heat treatment losses were evident. This study aims to investigate how the slicing and frying applications and storage of yellow onion products at different temperatures, light and modified atmosphere conditions, effect the flavonoid content and profile. By this means, the storage conditions retaining the flavonoids in packaged yellow onion products will be determined.