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

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