In a large bowl, break up the tuna into smaller pieces.
Mix in the red bell pepper, red onion, and cucumber. Pour the sesame oil on top and stir to combine everything. Lastly add the juice of 1 lime and stir again to combine.
Salt and pepper to taste and garnish with sesame seeds before serving.
Notes
If you feel more sesame oil is needed, feel free to add more to your taste!