Place strained corn kernels, finely diced onion and strained tuna in a mixing bowl flaking tuna with a fork and season with salt and pepper well.
Stir to combined.
On a stove top in saucepan over med heat melt butter combine flour till resembles wet sand and comes together. Add half the brine (discard the rest) stir till thickened.
Gradually add milk stirring till smooth before adding more milk. Repeat till all milk combined. Keep stirring and add in half cup cheese, stir till melted. Add tuna, corn and onion mix into sc.
Pour cheese sauce over and stir till combined.
Top with either cornflakes or bread crumbs and grated cheese.