Combine the tuna, mayonnaise, celery, red onion, mustard, parsley, salt and pepper in a mixing bowl.
Lightly butter one side of each slice of bread.
With butter side down, place a slice of cheese on half the bread slices.
Top cheese with tuna mixture, on each, followed by a slice of bread, butter-side up.
Place the sandwiches on a griddle, or in a frying pan, on medium heat; heat ~3 minutes, until cheese starts to melt and bread is golden brown; flip; cook ~3 minutes more.
Nutrition
Nutrition Facts: Servings: 8; Amount per serving: Calories 612; Total fat 21.2g 32.6%; Saturated fat 7.3g 36.5%; Cholesterol 181mg 60.3%; Sodium 637mg 26.5%; Carbohydrates 28.4g 9.5%; Fiber 1.4g 10.0%; Sugars 8.5g 17.0%; Protein 64.5g 129.0%; Based on DV of 2000 calories per day