Boundary-layer flashback is a major risk for lean-premixed combustion of H2-based fuels or fuel blends in low-emission gas turbines. This study demonstrates and critically assesses a methodology based on large eddy simulation (LES) coupled with conjugate heat-transfer (CHT) to predict boundary-layer flashback in turbulent H2 flames without ad hoc parameters. By coupling LES with CHT, we determine flashback limits as observed experimentally on a backward-facing step while avoiding the use of ad hoc thermal boundary conditions. The sensitivity of predictions to modeling and numerical parameters is assessed. It is demonstrated that the model presented is insensitive to external thermal boundary conditions applied on the outer surfaces of the solid parts. Accurate representation of the wall heat-flux in the immediate vicinity of the flame, proper flame resolution, as well as Soret diffusion are identified as key parameters for modeling. In particular, Soret diffusion locally enriches the boundary layer whenever the fuel/air mixture is preheated by heat exchange with solid components. Consequently, neglecting Soret diffusion leads to a nonconservative misprediction of the flashback limit. Flame thickening results in excessive heat loss in the flame zone and consequently leads to a systematic misprediction of the flashback limit. The proposed LES+CHT framework gives insight into the relationship of heat transfer, mixture preheating, and flame motion. The methodology is well-suited to predict thermal runaway limits, where the flame seems stable, but flashes back after sufficiently long operation. The differences between short-term flashback and thermal runaway, and the consequential difficulty of consistent validation with experiment are discussed.