A simultaneous turbo hydrodistillation (THD) extraction of essential oil (EO) and flavonoid contents from onion (Allium cepa L.) was studied. Optimization of EO extraction was performed, and the effectiveness of THD in the extraction of onion total phenol content (TPC), total quercetin (TQ), quercetin aglycon (QA), quercetin-3,4'-diglucoside (QDG), quercetin-4'-monoglucoside (Q4G), quercetin-3-monoglucoside (Q3G), and kaempferol (KMF) was evaluated and compared with conventional solvent extraction of fresh onion. THD offers valuable extraction of onion by-products: 68.90 g/t of EO, 95.86 % of total phenol, and 55.94 % of total quercetin contents. The conventional extraction of fresh onion allowed the presence of QDG (684.78 mg/100 g dry weight (DW)) and Q4G (423.44 mg/100 g DW), whereas QA, Q3G, and KMF were also present in onion by-products obtained by THD. At 180-min THD process, the highest phenol content was obtained in the aqueous phase (47.58 mg quercetin equivalents/g DW). QDG (315.29 mg/100 g DW) and Q4G (335.05 mg/100 g DW) were mainly present in the aqueous phase, whereas QA (271.79 mg/100 g DW) was mainly present in the solid phase. Q3G and KMF were detected in minor quantities in onion extracts.