How do I pass a mutable list to a bundle?

You can do

//apply plugin: 'kotlin-android-extensions' 

//androidExtensions {
//    experimental = true
//}

apply plugin: 'kotlin-parcelize'

Then

@Parcelize
class RecipeTemplate : Parcelable {
    var recipeHeader: String? = null
    var recipeText: String? = null
    var recipeImage: String? = null
    var recipeKey: String? = null
}

Then

var bundle = Bundle().apply {
                  putParcelableArrayList("LIST", ArrayList<Parcelable>(fbModel.recipeArray))
             }

This depends on what items you have in your list. Bundle has a couple methods that allow you to place lists in it, for example putIntegerArrayList and putStringArrayList. If you need to use a custom type, you can make it Parcelable and use putParcelableArrayList.

Of course, all these methods as their name suggest take ArrayList instances specifically, not just any MutableList. So you can either change all your MutableList usages to use ArrayList instead, or you can create a new ArrayList from your existing MutableList when you're putting it in the bundle:

bundle.putParcelableArrayList("key", ArrayList(myParcelableList))