For those who like specific measurements, I use a lightly rounded cup each of chopped bell pepper, halved cherry or grape tomatoes, and sliced cucumber, and about ¼ cup of slivered red onion.
What kind of tuna? Albacore tuna has a mild flavor and firm texture that holds up well in a salad. I usually buy skipjack tuna, however, because it has lower mercury and higher selenium levels than other types of tuna. Alternatively, canned salmon or even sardines may be used.
*Pro tip: We like to use half a yellow bell pepper and half an orange one. The tomatoes supply the red, and the combination creates an extra pretty, garden-like appeal.