Structural stability of SARS-CoV-2 3CLpro and identification of quercetin as an inhibitor by experimental screening – ScienceDirect
— Read on www.sciencedirect.com/science/article/pii/S0141813020339970

This is on top of Quercetin also being a Zinc ionophore, which helps increasing the amount of intracellular elemental zinc, which in turns inhibits replication of RNA viruses.