The fast brightness rise in FU Ori has been an 80+ year long mystery. We examine whether an unbound stellar flyby can initiate FU Orionis outbursts using 3D hydrodynamics simulations coupled to live Monte Carlo radiative transfer. We find that a flyby where the secondary penetrates the circumprimary disc triggers a 1-2 year rise in the mass accretion rate. Importantly, we find that the less massive star becomes the dominant accretor, similar to the outburst observed in FU Ori.