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

{
  "parent": "minecraft:block/slab",
  "textures": {
    "bottom": "minecraft:block/red_nether_bricks",
    "side": "minecraft:block/red_nether_bricks",
    "top": "minecraft:block/red_nether_bricks"
  }
}