1. Stretch aluminum foil over an oven tray, place sweet potatoes, sage sprigs and butter on top, drizzle maple syrup, and add salt and pepper liberally, then cover all with more foil, seal the edges and bake for two hours at 300 degrees.
2. Pull out of the oven, remove sage, peel sweet potatoes, and transfer them to a blender along with the juices left in the foil (use a spatula if necessary).
3. Blend three to four minutes until smooth, adding a bit of water if needed to achieve a creamy texture, then transfer to a saucepan and set aside.
4. Spread flour on a dish, drop a cobia fillet to lightly coat both sides, heat canola oil in a sauté pan, lower heat, add the fillet and cook eight to 10 minutes (until brown on one side), checking for doneness before removing from heat.
5. Warm up the sweet potato puree, stir until smooth and set aside.
6. Pour out the oil from the sauté pan, add butter, and baste fish until butter starts to brown, flipping the fillet in the process. Remove from heat, add capers, walnuts, parsley and lemon juice, and check for salt.
7. To plate, serve sweet potato puree, place cobia fillet on top, and spoon sauce from the pan over the fish.