rndpack/assets/minecraft/models/block/mossy_cobblestone_slab.json

{
    "parent": "block/slab",
    "textures": {
        "bottom": "block/mossy_cobblestone",
        "top": "block/mossy_cobblestone",
        "side": "blocks_extra/slabs_stairs/mossy_cobblestone_slab_stair"
    }
}