Mix the soy sauce, horseradish, and maple syrup in a small bowl and transfer to a zip lock bag. Add the tuna steaks and toss well to coat evenly and refrigerate for an hour.
Meanwhile, mix 20g fresh horseradish, grated, 60ml sour cream, 0.25tbsp white wine vinegar,1tbsp Dijon mustard, 2 pinch salt, 2 pinch black pepper (freshly ground) in a mixing bowl and whisk continuously until you get a smooth and creamy consistency. Transfer the sauce to an air tight container and allow to rest in a refrigerator for at least 4 hours for the flavors to marry. Note: you can prepare the sauce prior to making this recipe.
Remove the tuna and grill on an outdoor grill over high heat for 2-3 minutes (depending on how you like your tuna done) per side with the grill lid on.
Place the cooked tuna on a serving platter and drizzle with the horseradish sauce.