Tuna and onion takes the bound tuna baseline and sharpens it with raw bite. The tinned tuna and its measured mayonnaise bind are the constant; the variable is raw onion, usually red, and it is the most aggressive answer in the whole family. Where cucumber cools and sweetcorn softens, onion attacks: a pungent, sulphurous, lingering sharpness that cuts straight through the richness of the mayonnaise and the saltiness of the fish and leaves the sandwich tasting bright and slightly hot. This is the version for people who find plain tuna mayo too mild and bland, and the onion is not a garnish here but the corrective the whole sandwich is built around.
The craft is balancing that bite so it sharpens rather than dominates. The onion is diced fine rather than sliced, so its heat is distributed evenly through every forkful instead of arriving in raw, overpowering slabs, and red onion is preferred because it is milder and sweeter than a brown one and stains the filling less. Quantity is the real discipline: a measured amount lifts the tuna, while too much turns the sandwich into onion with tuna attached and leaves a taste that outlasts lunch by hours. The tuna is bound to the same just-cohesive point as the baseline, the bread is soft and plain, and the filling is spread evenly so the bite is consistent rather than concentrated in one corner. Onion carries no extra water, so unlike cucumber it does not add a moisture problem, only an intensity one.
The variations are the rest of the tuna family, each leading on a different counter to the same fixed bind: cucumber for cool water-crisp, sweetcorn for a sweet pop, celery and pepper for the branded crunch, plain mayo for the baseline. Onion is the sharp one, and spring onion or a milder white onion are the quieter settings of the same idea. Each deserves its own article rather than being crowded in here.