Combine all of the ingredients for the salsa in a bowl. Gently stir with a spoon to ensure its well mixed. Place in the fridge until ready to use.
To prepare the chipotle sauce, combine all of the chipotle cream ingredients listed in a high speed blender (such as a nutribullet) and blend until completely smooth. You may need to add more water to get the liquidy consistency of a sauce. It will slightly thicken when stored in the fridge.
In a rimmed dish whisk together the orange and lime juice, oil and garlic. Place the tuna steaks in the dish, sprinkle with salt and let marinate for 10 minutes per side.
To prepare the tortillas cut both ends off of the celery root and then slice off the outer peel. Using a sharp knife cut the celery root into slices that are as thin as possible (watch the video above for a demonstration on how to cut the tortillas)
Preheat the BBQ to medium heat or alternatively heat a grill pan on the stove on medium heat. Once hot grill the tortillas for approximately 4 minutes per side until grill marks form and the tortillas are tender and pliable.
Once the tortillas are done cooking, place the tuna steaks on the grill and cook for just 2 minutes per side (cook longer if you prefer your tuna more well done)
Transfer the tuna steaks to a cutting board and cut across the grain into 1/4-inch-thick slices.
To prepare the tacos, top each of the tortillas with some shredded red cabbage, avocado, jalapeño, 2 slices of the tuna and a spoonful of the salsa and chipotle sauce.