In a small bowl mix together: tuna, celery, red onion, mustard and mayonnaise. Add fresh pepper, taste and adjust seasonings.
Toast the bread to a light brown.
Top with tuna salad, then grated cheese.
Finish with a slice of tomato.
Place in a toaster oven and toast until the cheese is melted 1-3 minutes. (or place in the oven and broil).
Garnish with fresh basil if desired.