With a slicing attachment on a food processor, slice the sweet potatoes and apples in two batches. You will need to cut everything to fit in the feeding tube of the food processor. Mix apples and sweet potatoes in a bowl with salt and ¼ cup olive oil.
Then slice the onions in food processor. Put 1/3 onions in a bowl and the other 2/3 onions in a hot cast iron skillet with olive oil and sauté until browned (about 20 minutes)
In a deep dish (9”x13”) coat bottom with 2 Tbls of olive oil Then add ½ of the apple sweet potato mixture. Next, add the onions (the 1/3 of the onions that did not get sautéed). Top with remaining mixture of sweet potatoes and apples and lightly press down layer. Pour coconut milk evenly over the entire casserole.
When onions are a deep golden brown, remove from heat and carefully spread over top of casserole. I used a spatula to spread mine.
Cover with tin foil and bake at 350 degrees for 90 minutes. Take tin foil off for last 15 minutes. Test with knife in middle for doneness. Let sit for ten minutes so the casserole can absorb juices before serving.