Peel onion and cut into rings. Drain the corn in a sieve. Drain tuna as well. Wash parsley, dab dry, remove leaves and cut finely. Heat half the butter in a large pan. Fry 2 slices of toast in it until crispy, take them out. Heat the rest of the butter in the pan, fry 2 more slices of toast until crispy, remove.
Spread each slice of toast with 1 1/2 tablespoons of cream cheese. Cover with tuna and onion rings. Sprinkle with corn, parsley and pepper