minecraft-assets/data/minecraft/recipes/composter.json

{
  "type": "minecraft:crafting_shaped",
  "pattern": [
    "# #",
    "# #",
    "###"
  ],
  "key": {
    "#": {
      "tag": "minecraft:wooden_slabs"
    }
  },
  "result": {
    "item": "minecraft:composter"
  }
}