Preheat oven to 350º. In a microwave-safe dish, bring the milk to a boil. Remove the milk from the microwave and add the cornmeal. Stir and let soak at room temperature for 30 minutes. Set aside.
Melt the butter in a 10-inch cast-iron skillet over medium heat. Once the butter has melted, add the brown sugar and stir until the sugar dissolves, about 2 minutes. Remove the skillet from the heat and carefully place 1 slice of pineapple in the center of the pan. Place the other 5 slices around the center slice in a circle. Place the cherries in the center of the pineapple slices and sprinkle the nuts evenly over the fruit. Drizzle the reserved 3 TBS. of pineapple syrup over top.
Sift the flour, baking powder and salt into a medium mixing bowl and whisk to combine. In a separate mixing bowl, whisk the eggs. Add the sugar to the eggs and whisk to combine. Add the canola oil and whisk. Add the cornmeal and milk mixture and whisk to combine. Add this to the flour and stir just until combined. Pour the batter over the fruit in the skillet and bake for 35 to 45 minutes, until a cake tester comes out clean.
Remove from oven and let cool for about 30 minutes in the skillet. Set a platter on top of the skillet and carefully invert the cake.