  Moir\'e superlattice in twisted bilayer graphene has been proven to be a
versatile platform for exploring exotic quantum phases. Extensive
investigations have been invoked focusing on the zero-magnetic-field phase
diagram at the magic twist angle around $\theta=1.1\degree$, which has been
indicated to be an exclusive regime for exhibiting flat band with the interplay
of strong electronic correlation and untrivial topology in the experiment so
far. In contrast, electronic bands in non-magic-angle twisted bilayer graphene
host dominant electronic kinetic energy compared to Coulomb interaction. By
quenching the kinetic energy and enhancing Coulomb exchange interactions by
means of an applied perpendicular magnetic field, here we unveil gapped flat
Hofstadter subbands at large magnetic flux that yield correlated insulating
states in minimally twisted bilayer graphene at $\theta=0.41\degree$. These
states appear with isospin symmetry breaking due to strong Coulomb
interactions. Our work provides a platform for studying the phase transition of
the strongly correlated Hofstadter spectrum.
