  We use a lowest Landau level model to study the recent observation of an
anomalous Hall effect in twisted bilayer graphene. This effective model is
rooted in the occurrence of Chern bands which arise due to the coupling between
the graphene device and its encapsulating substrate. Our model exhibits a phase
transition from a spin-valley polarized insulator to a partial or fully valley
unpolarized metal as the bandwidth is increased relative to the interaction
strength, consistent with experimental observations. In sharp contrast to
standard quantum Hall ferromagnetism, the Chern number structure of the flat
bands precludes an instability to an inter-valley coherent phase, but allows
for an excitonic vortex lattice at large interaction anisotropy.
