Heat the oven to 350˚F. Thinly slice the sweet potato, ideally using a mandolin. I slice mine on the thinnest setting of my small tabletop mandolin. Thickly slice the apple into ¼” thick pieces using a sharp knife.
Melt the butter, coconut sugar, cinnamon, cardamom, nutmeg, and salt together. (I zap mine in a microwave safe dish for 30 seconds, and stir.)
Arrange two layers of thin sweet potato slices in the small casserole dish. Spoon over ½ of the spicy butter. Arrange the sliced apples and onions on top and dot with gorgonzola. Layer the remaining sweet potato slices on top. Pour the half and half over the layers.
Stir the oats and pepitas into the remaining spicy butter and spoon onto the top of the casserole.
Cover with plastic wrap and microwave on high for 3 minutes. Remove the plastic and slide it onto a baking sheet and bake in the oven for 15 minutes. The potato slices should be fork tender, and the casserole should be bubbly hot.
Serve warm.