Future long-baseline neutrino oscillation parameter measurements will be dominated by systematic rather than statistical uncertainty. The determination of incident neutrino energy based on experimental observables relies on theoretical neutrino-nucleus interaction models, which is the dominant uncertainty for the HK experiment. E61 is an experimental solution to constrain this uncertainty. E61 is a water Cherenkov detector, constructed using multi-PMT modules, that can be raised/lowered to span a 1-4 degree off-axis range. Linear combinations of the near detector flux, at different off-axis angle slices, can be taken to match the far detector flux measurement. Oscillation parameters may then be extracted while largely reducing neutrino interaction model dependence. E61 also enables unique measurements of neutrino cross sections as a function of neutrino energy, can measure far detector intrinsic backgrounds, and neutron emissions from neutrino interactions by doping with Gadolinium.