A novel topology of electronically-steerable one-dimensional Fabry-Perot leaky-wave antenna (LWA) is proposed. The structure is based on a host parallel-plate waveguide (PPW) which encloses a Fabry-Perot cavity created by two printed-circuits boards (PCB): a top planar partially reflective surface (PRS) and a bottom tunable high impedance surface (HIS) loaded with varactors. The control of the scattering properties of the bottom tunable HIS by the variation of the varactors' capacitance enables control of the Fabry-Perot cavity resonance and thus the scanning of the antenna pointing angle from broadside towards the endfire direction at a fixed frequency. Full-wave simulations obtained with commercial finite element method solver (HFSS) demonstrate as a proof of concept a continuous scanning range of the pointing angle from 5° to 50° at a design frequency of 5.6 GHz.