pumpkin-java-multiversion/assets/datapacks/26_3/data/minecraft/recipe/composter.json

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