1. In a mixing bowl, combine the sour cream, mayonnaise, and a squeeze of lime or lemon juice. Whisk until you have a smooth, creamy sauce. Set the sauce aside or store in the refrigerator if you're making it more than 15 min ahead of time.
2. In another bowl, stir together your chopped onions and finely chopped cilantro until evenly mixed.
3. Prepare cherry tomatoes, radish and fresh chilli.
4. Heat a teaspoon of vegetable oil in a cast iron skillet. Once it's hot, sauté the tuna. You're aiming to slightly brown it, not blacken it.
5. Prepare your tortillas by warming them briefly on both sides in a clean pan or skillet – no oil required.
6. To assemble your tacos, lay them flat on a clean work surface or individual plates. Onto each one, add the tuna then the onion and cilantro mixture. Finally, top with cherry tomatoes, radish, fresh chilli and creamy sauce.