On 19–20 August 2024, the European Space Agency (ESA) Jupiter Icy Moons Explorer (JUICE) mission performed the first ever Lunar–Earth Gravity Assist (LEGA) manoeuvre. The mission was launched on April 14, 2023, and is currently on an 8-year interplanetary cruise to the Jovian system. It is equipped with the RADiation-hard Electron Monitor (RADEM), a facility instrument designed to measure the most energetic particle populations (electrons, protons and ions) in the Jovian environment where JUICE will operate. During LEGA, JUICE crossed the Van Allen belts, providing a unique opportunity to evaluate the in-flight response of RADEM and to optimize its configuration for the Jupiter phase. In this paper, we report RADEM observations of the Van Allen belts, showing clear sensitivity to trapped electrons and protons. We also discuss how the Earth-flyby geometry, including pitch-angle effects, influenced the measurements and the implications for future operations. The observations also demonstrate that while RADEM is a facility instrument, it has the potential to enhance the scientific return of the JUICE mission by monitoring a key energy range in Jupiter's radiation belts that no other instrument on JUICE is covering.