The residue levels of the fungicide, boscalid on Welsh onions after cooking via different methods, were investigated. After boscalid treatment, Welsh onions of varying sizes were cooked via stir-frying, baking, and boiling under different heating times, and residue quantification was realized via LC-MS/MS (limit of quantification, 0.01 mg/kg
recovery rate, 84.6-102.7%). Residues levels after stir-frying and baking were 1.81-2.28 (processing factor (PF), 0.70-0.88) and 0.57-1.16 (0.22-0.45) mg/kg, respectively. Boiling 0.8 and 5 cm slices resulted in residue levels of 0.49-0.88 (0.19-0.34) and 0.23-0.58 (0.09-0.22) mg/kg, respectively, and the PF for baking and boiling decreased with increasing heating time, indicating an effective reduction of residue level by heat. However, stir-frying showed increased residue levels with extended heating owing to water loss. Thus, boiling was the most effective method in reducing pesticide residue levels in Welsh onions.