heck/kubejs/server_scripts/recipes/essentials/slabs.js

// priority: 0

onEvent("recipes", event => {
  // For each slab crafting recipe that outputs 6x slabs, ...
  event.forEachRecipe({ id: /slab/, type: "minecraft:crafting_shaped" }, recipe => {
    if (recipe.outputItems[0].getCount() != 6) return;

    let input  = recipe.inputItems[0];
    let output = recipe.outputItems[0].getId();

    // We disable the Immersive Engineering storage block slabs in "immersive_engineering.js".
    if (/^immersiveengineering:slab_storage_/.test(output)) return;
    // Quark limestone is disabled but some recipes still linger.
    if (/^quark:.*limestone.*$/.test(output)) return;

    // ... add a recipe to craft slabs in 2x2 crafting grid.
    event.shaped("4x " + output, [ "BB" ], { B: input });
    // For this to work, "pressure_plates_use_stabs.json" must also be present.

    // ... add a back-crafting recipe to turn 2x slab => 1x block.
    event.shaped(input, [ "S ", " S" ], { S: output });
  });
});