Preheat oven to 350°F. Oil a large circular dish with coconut oil. Set aside.
Make the apple topping. Chop 3 apples into ½" chunks. Add ghee to a medium-sized saucepan & heat over medium heat. Once melted, add the apples. Toss together & soften for 1-2 minutes. Next, add the coconut sugar, sea salt, vanilla, & cinnamon & mix together. Using a fork, continue mixing & mashing the apples until soft & well broken down. This will take about 5 minutes. Once finished, turn off the stovetop & add the lemon juice. Stir together & set aside.
Prepare the cake. If making your own oat flour, begin by blending 1/4 c oats into a fine flour. Skip this step if using ready-made oat flour.
In a medium bowl, add all of the dry ingredients, including the oat flour, & mix together.
In a separate medium bowl, whisk together the eggs & sugar until thoroughly combined. Next, melt the coconut oil & add to the extra mixture along with all of the remaining wet ingredients. Stir together until there are no clumps remaining.
Slowly pour the dry ingredients into the wet mixture, mixing as you go. Stir together until there is no dry flour remaining.
Grab your baking dish & pour the batter in. Set aside.
Prepare the crumb topping. In a small dish, mix the flour, oats, sugar, cinnamon, & sea salt. Add the room-temp coconut oil, it should be soft & malleable, not melted. Incorporate the mixture into the coconut oil using your hands, continue until you have a crumbly consistency.
Add your toppings. Grab your baking dish & begin to top the cake with your apple mixture. Place one layer of the mashed apples on top of the batter, leaving ½ - 1 inch of cake batter uncovered around the entire edge of the dish. Take your crumb topping & do the same thing, placing it over the apples. As the cake bakes, the toppings will sink to the center of the cake & the edges will come in, covering the apple & oat mixture.
Decorate the top. Though not required, this step is a nice touch. Slice the remaining 1/2 apple as thin as possible. Arrange however you'd like across the top of the cake, tucking one end into the cake batter to hold them in place. If you're looking for inspiration, I like to stack 4-5 slices together then fan them out & place on top. Repeat this 3-4 times.
Finally, place in the oven to bake for 50 minutes. Check on the cake at 45 minutes, take it out of the oven when the toothpick (or knife) comes out just barely clean.