Abstract
We construct a semi-inclusive Lepton Flavour Universality (LFU) ratio, $R_\Sigma$, to test $\mu/e$ universality in $b\to s\ell^+\ell^-$ transitions at $e^+e^-$$B$-meson factories. Combining different decay channels, this observable maximises the sensitivity to possible LFU violations of short distance origin, yet preserving a clean theoretical interpretation in case of a deviation from its Standard Model prediction, $R_\Sigma^{\rm SM}=1$.