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

{
  "type": "minecraft:crafting_shaped",
  "key": {
    "M": {
      "item": "minecraft:melon_slice"
    }
  },
  "pattern": [
    "MMM",
    "MMM",
    "MMM"
  ],
  "result": {
    "item": "minecraft:melon"
  }
}