This study evaluated the effects of inorganic fertilizers on growth, yield, bulb quality, and optimal fertilizer rates in onion (Allium cepa L.) cultivation in the Mid-Hills of Himachal Pradesh. A field trial in 2021 tested nine treatments with varying recommended doses (RD) of nitrogen (N), phosphorus (P), potassium (K), and sulphur (S). Significant differences were observed in plant height, fresh bulb weight, marketable yield, bulb diameter, nutrient content, and total soluble solids (TSS) across treatments. The T₂ treatment [100% RD (P, K, S) + 125% N] produced the highest plant height (51.9 cm), fresh bulb weight (88.2 g), yield (310 q/ha), and TSS (12.53°B), closely followed by T₈ [100% RD (N, P, K) + 125% S]. Yield improvements of 13.88% (T₂) and 10.17% (T₈) were recorded over the control. The highest plant nitrogen (2.67%), phosphorus (0.37%), and sulphur (0.45%) contents were observed in T₃, T₄, and T₈, respectively. Results highlight the critical roles of nitrogen and sulphur in enhancing productivity and the risks of nutrient imbalance. For sustainable production, applying 100% RD of P, K, and S with 125% N (T₂), or 100% RD of N, P, and K with 125% S (T₈) is recommended. Future research should integrate these findings with precision farming to improve nutrient-use efficiency and minimize environmental impact.