The fission response function (FRF) method–based neutronics code FLASH is formally introduced in this work. FLASH adopts a two-step lattice-to-core approach. The first step involves generating a database of FRFs and environmental factors and the second step constructs the global core fission matrix using this database and solves for its eigenpairs. This paper systematically presents the architecture, theoretical foundations, and computational workflow of FLASH, including the FRF generation, core calculation algorithms, environmental factor modeling, online power reconstruction, and kinetics capabilities. The code has been rigorously validated against a variety of benchmark problems, and several representative cases are highlighted. FLASH is capable of performing high-fidelity, full-core static and transient neutronics simulations within minutes, demonstrating significant potential as an efficient and accurate hybrid neutronics method. Prospective improvements and ongoing developments are also discussed.