Friends will wonder how in the world these clever -- not to mention edible -- containers were created. The answer: a melon baller.
Slice cucumbers into 3/4-inch-thick rounds; use a small melon baller to scoop out some seeds to make a cup. Mix tuna, oils, Sriracha, and a generous pinch of salt; spoon into cups. Combine shallot and lime; sprinkle over tuna mixture. Serve immediately.