VerticalSlabs/src/main/resources/data/verticalslabs/recipes/warped_slab.json

{
    "type": "minecraft:crafting_shapeless",
    "ingredients": [
        {
            "item": "verticalslabs:vertical_warped_slab"
        }
    ],
    "result": {
        "item": "minecraft:warped_slab"
    }
}