Place vinegar in coconut milk and set aside.
In a large bowl, beat together eggs, honey, and sugar with a mixer until the sugar is well dissolved. Add avocado oil and beat well for another minute, then stir in vanilla, cream of tartar, baking soda, cinnamon and salt. Pour in the coconut milk and stir to combine. Add cassava flour, coconut flour, and tapioca starch into the wet ingredients and stir well to combine, then fold in the apple.
Pour about 1/2 of the batter into the prepared pan and then add in about half of the filling mixture over top. Repeat again with the remaining batter.
Use a thin knife to make a few swirls in the batter if you'd like more swirls in the cinnamon filling.