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Glycemic variability is a stand-alone risk factor for diabetic complications. Background/Objectives: We investigated whether combining quercetin-rich Allium cepa L. (white onion) juice with the sodium-glucose cotransporter-2 (SGLT2) inhibitor dapagliflozin could reduce glycemic variability beyond that achieved with monotherapy in experimental diabetes. Methods: Wistar rats were made diabetic with streptozotocin (35 mg/kg) and maintained for 60 days in the following groups: vehicle, dapagliflozin (0.1 mg/day), fresh onion juice (5 mL twice daily), and dapagliflozin + fresh onion juice (5 mL twice daily). Diabetic Wistar rats (n = 10/group) were gavaged with vehicle, dapagliflozin (0.1 mg/day), fresh onion juice (5 mL twice daily), or both. The juice was expected to contain phytochemicals (quercetin derivatives and organosulfur compounds) based on published reports on similar A. cepa cultivars. A validated immunoassay was used to measure glycated hemoglobin (HbA1c) every 10 days. All treatments reduced HbA1c to the same level (~7.1-7.2%) as compared to diabetic controls (9.0 ± 0.3%) (p < 0.001), though only the combination treatment reduced glycemic variability (HbA1C coefficient of variation 3.9 ± 0.6% vs. 7.8 ± 1.2% with dapagliflozin and 11.2 ± 1.8% with onion) (p < 0.001). Results: Across the 10-day sampling schedule, the combination kept HbA1c within a narrow 6.8-7.2% band, whereas the monotherapies fluctuated more widely; because intraday and postprandial glucose were not captured, effects on short-term excursions could not be directly assessed. There were no cases of severe hypoglycemia and only infrequent and non-repeated cases of mild hypoglycemia. Conclusions: In this exploratory, hypothesis-generating study, the additive interaction between onion phytochemicals and dapagliflozin was associated with lower glycemic variability via mechanisms proposed in the literature; quercetin-rich A. cepa juice may therefore warrant further investigation as an adjunct in diabetes management, pending batch-specific phytochemical characterization and confirmatory studies.