MineAssets/1.12.2/assets/minecraft/recipes/cobblestone_slab.json

{
  "type": "crafting_shaped",
  "pattern": [
    "###"
  ],
  "key": {
    "#": {
      "item": "minecraft:cobblestone"
    }
  },
  "result": {
    "item": "minecraft:stone_slab",
    "data": 3,
    "count": 6
  }
}