Grease a 9 inch round baking dish with oil and line the bottom with a circle of parchment paper. Preheat the oven to 375 degrees F (177 C).
Cut a plum into quarters and cut each quarter into about 3-4 small slices so that you end up with 14-16 even slices. Repeat with two more plums. Place the slices in the bottom of the pan to see if you’ll need to slice up one more in order to cover the bottom of the pan. If you have enough slices or more than enough, place all of them in a small saucepan. If you don’t have enough slices or just barely enough, slice up the last plum because cooking them makes them shrink a little.
In the saucepan with the plum slices, add the maple syrup, oil, vanilla, and salt. Cook on medium-low heat for about 4-5 minutes until the liquid looks slightly more syrupy. Pour the plums and all of the juicy syrup into the prepared baking pan, arranging the slices so they look nice.
In a large bowl, whisk the oat flour, almond flour, cornmeal, sugar, salt, baking powder, and baking soda together. Add the pumpkin purée, oil, lemon juice, vanilla, almond extract, and water and whisk until well combined and smooth. Gently pour the batter over the plums, scraping the sides of the bowl with a spatula to get every last bit of batter. Bake for 30-40 minutes or until a toothpick comes out clean. Let it cool in the pan at room temperature for about an hour.
After an hour, loosen the edges of the cake with a knife and invert the cake out onto a plate. Remove the parchment circle and admire the beautiful plum artwork! Slice, serve, and enjoy!!
