For the ginger pickled carrots: Combine the sugar, vinegar and 2 cups water in a pot and bring to a boil, stirring until the sugar is dissolved. Pour into a plastic container and let cool completely.
Once cooled, add the carrots and ginger to the pickling liquid¿make sure they are completely covered by the liquid. Refrigerate for at least 6 hours before using (24 hours is best).
For the sweet potato puree: Preheat the oven to 350 degrees F. Pierce the skins of the sweet potatoes and bake until tender, about 45 minutes. (Leave the oven on for heating up the tortillas.)
Once cooked, cut the sweet potatoes in half and scoop the flesh into a food processor. Add the butter and honey and puree until smooth. Season with salt and pepper. Hold hot until ready to plate.
For the tuna: Combine the Cajun seasoning and salt, and mix. Sprinkle the tuna pieces with the spice mixture. Heat a large saute pan over high heat until the pan begins to smoke. Add the grapeseed oil to the pan and then sear the tuna on two sides, leaving it rare in the center. Remove the tuna from the pan and let rest for 5 minutes.
For the guacamole: Scoop the avocado flesh into a bowl. Add the red onions, tomatoes, cilantro and lime juice, and mash with a fork to combine. Season with salt and pepper.
To build the taco: Heat the tortillas in the oven just until warm, not crisp, 2 to 3 minutes. Spoon 2 tablespoons of the sweet potato puree onto each tortilla. Add a piece of tuna, then top with about 1 ounce of the pickled carrots and 1 tablespoon of the guacamole. Garnish with two sprigs of cilantro and serve.