On September 9, 2021, the Hauts-de-France Regional Health Agency (ARS) was notified of a suspected outbreak of foodborne illness affecting day students attending several schools in a municipality within the Lille metropolitan area. On September 13, 2021, two cases of hemolytic uremic syndrome (HUS) were diagnosed in two hospitalized children attending school in this municipality. Santé publique France Hauts-de-France was contacted by the ARS of Hauts-de-France to provide support for the investigation and management of this foodborne illness outbreak. In total, 35 cases of gastroenteritis, with bloody diarrhea and fever (>38°C) in half of the cases, were identified. Ten cases were hospitalized, and two children developed HUS. The identified cases included day students in four school groups (29 cases), a parent of a student, and elderly individuals receiving the municipality’s home meal delivery service (5 cases). The cafeterias of the affected cases were all supplied by the municipal central kitchen. The pattern of the outbreak curve suggested a common, single source of contamination during meals on September 2 or 3, 2021. The case-control investigation, conducted in the schools, concluded that only the consumption of cucumbers in a salad, served at the meal on September 2, was statistically and significantly associated with the onset of the illness. A highly pathogenic Shiga toxin-producing E. coli (STEC) O157 strain was isolated from stool cultures of eight cases, including the two children who developed HUS, and from the implicated cucumber salad. Genomic analysis of the strains confirmed that the clinical and foodborne strains belonged to the same genomic cluster. The veterinary investigation revealed that a failure in the decontamination process, combined with incomplete peeling of the contaminated cucumbers, contributed to the outbreak of this foodborne illness. The implicated cucumbers originated in Belgium, and Belgian health authorities were notified via the dedicated European alert systems. No other cluster of STEC infection cases linked to this foodborne illness outbreak was reported to the ARS during the period in question, even though cucumbers from the same batch had been widely distributed to institutions and commercial food service establishments in the Hauts-de-France region. The food vehicle implicated in this foodborne illness outbreak is classified as a high-risk vegetable due to its consumption as a raw food. It is important to remind vulnerable populations and institutional food service providers that preventing the risk of STEC infection associated with the consumption of raw vegetables requires washing, disinfecting, and peeling.