minecraft-assets/assets/minecraft/models/block/smooth_red_sandstone_slab_top.json

{
  "parent": "minecraft:block/slab_top",
  "textures": {
    "bottom": "minecraft:block/red_sandstone_top",
    "top": "minecraft:block/red_sandstone_top",
    "side": "minecraft:block/red_sandstone_top"
  }
}