New exciting results have been published by the LHCb experiment on the Lepton Flavour Universality (LFU), especially in rare loop-mediated processes. To further corroborate or discard New Physics (NP) scenarios, additional testing is needed at fundamental tree-level processes: baryonic semi-leptonic decays provide a unique means to test for LFU at the LHCb, given e.g. their production abundance and favourable BR. In addition, they exhibit enhanced sensitivity to NP in the angular observables of the decay products. I present the angular analysis of the process $\Lambda_b \to \Lambda_c \mu \nu$ as a function of the squared di-lepton invariant mass and lepton helicity angle.