Place the sushi-grade tuna on a clean cutting board. Use a very sharp knife to dice it into small, even cubes. Work gently to keep the texture smooth and fresh.

In a mixing bowl, combine soy sauce, olive oil, sesame oil, Dijon mustard, and lemon juice. Stir well until the mixture looks smooth and slightly thick. This will flavor the tuna.

Add chopped capers, red onion, and chives to the dressing. Stir gently to combine. These ingredients bring a fresh, slightly tangy taste that balances the rich tuna.

Carefully add the diced tuna into the bowl. Mix gently using a spoon. Do not overmix. You want the tuna pieces to stay intact and soft.

Add salt and black pepper to taste. Mix lightly again. Taste the mixture and adjust if needed. A little extra lemon juice can brighten the flavor.

If using avocado, fold it in gently at the end. This adds a creamy texture and mild flavor. Be careful not to mash the avocado while mixing.

Use a serving ring or small bowl to shape the tartare on a plate. Press lightly to hold the shape. Carefully lift the ring for a clean presentation.

Serve the tartare right away for the best freshness. You can add a drizzle of olive oil or extra chives on top for a simple and elegant finish.