In a medium bowl, whisk together the olive oil, soy sauce and lime juice.
Add the diced tuna, cucumber, capers, sesame seeds, olives and cilantro. Carefully toss to combine.
Divide the tartare between 4 chilled serving bowls and serve with tostadas.
For easier slicing, freeze the tuna for 15 minutes.
Recipe courtesy of Marcela Valladolid