Flat bands (FBs), presenting a strongly interacting quantum system have drawn increasing interest recently, as partly reflected by its discovery in twisted bilayer graphene in association with superconductivity. However, experimental realization of topological FBs has been hampered by lacking FB materials, and especially remained elusive in the ideal platform of monolayer materials where they arise from destructive quantum interference as predicted in 2D lattice models. Here, we report experimental synthesis of mesoscale ordered self-assembled monolayer of 2D hydrogen-bond (H-bond) organic frameworks (HOFs) of 1,3,5-tris(4-hydroxyphenyl)benzene (THPB) on Au(111) surface and the observation of related topological FB. Its formation, atomic and electronic structures have been characterized with a suite of experimental and theoretical techniques in excellent agreement. High-resolution scanning tunneling microscopy/spectroscopy (STM/STS) shows mesoscale, highly-ordered and uniform THPB-HOF domains, while angle-resolved photoemission spectroscopy (ARPES) highlights a FB over the whole Brillouin zone (BZ). Density-functional-theory (DFT) calculations and analyses reveal that the observed topological FB arises from a hidden electronic breathing-Kagome lattice without atomically breathing bonds. Our findings demonstrate that self-assembly of HOFs provides a viable approach for synthesis of 2D organic topological materials, paving the way to explore many-body quantum states of topological FBs.