With the introduction of carbon-free fuels and their rapid implementation, accurate combustion models are needed to design decarbonized engines, gas turbines or burners. Recent experimental and numerical work have highlighted the high burning rate of lean hydrogen and hydrogen/ammonia flames and their deviation from flamelet assumptions, pointing out the need to revise classical combustion models for Reynolds Averaged Navier-Stokes (RANS) or Large-Eddy Simulations (LES). In this work, a novel subgrid-scale model accounting for stretch effects for lean ammonia/hydrogen (NH3/H-2) flames in the Thickened-Flame LES framework (TFLES) is proposed as an extension to the standard Charlette model Charlette et al. (2002). This model, called ESE-NH3, is based on 1-D premixed strained flames and validated a priori with Direct Numerical Simulations (DNS) and used to perform LES simulation of a turbulent slot burner configuration. To assess the performance of the novel subgrid model, LES predictions are compared a posteriori to the DNS and to LES employing the standard Charlette model. The comparison shows that the LES computation with the novel subgrid-scale model accounting for stretch effects predicts flame lengths closer to the DNS. This shows that thermo-diffusive effects can be integrated simply in the TFLES framework. Moreover, results show that these effects are significant in all lean NH3/H-2 flames even at low H-2 contents.