For the dipping sauce: Mix together the ponzu, mirin, soy sauce and ginger in a small bowl. Add sambal oelek to taste. Set aside half of the sauce for brushing on the fish as it grills.
For the fish: Preheat a grill or grill pan to medium heat and brush with oil so the fish doesn't stick. Brush the fish on both sides with dipping sauce and grill for 20 to 25 minutes, flipping and brushing with sauce every 5 minutes. When done, the fish should be crispy and nicely charred and the flesh cooked through. The best way check for doneness is to insert the tip of a knife or a cake tester into the flesh, then pull it out and hold it to your wrist; it should feel warm. When it does, you're ready!
Mix the grated daikon and shiso in a small bowl.
Serve the fish with the dipping sauce and daikon mixture on the side. If you like, add some of the daikon mixture to the dipping sauce.