In this paper, we introduce a developed version of mushroom nanoantenna sensor array suitable for refractive index sensing. The single element structure consists of gold-silica disks placed on top of a silica film backed by a gold layer. The whole structure is supported by silicon substrate, which is a passive substrate shielded from electromagnetic fields. The presence of the ground plan allows for isolating the proposed sensor from the medium underneath, which make it immune against parasitic variations in the supporting medium. The proposed developed mushroom sensor is fully optimized using a rigorous optimization algorithm. It shows a figure-of-merit (FoM) as high as 579, which is significantly better than other versions of mushroom sensors already presented in literature. The proposed device is fabricated using Electron Beam Lithography (EBL).