In a bowl, combine tuna, apple, mayonnaise, onion, mustard and curry powder; mix well. Spread 1/4 cup on each slice of toast; sprinkle with walnuts.
Top with a slice of cheese. Broil 5 inches from heat until cheese is melted.
*To toast walnuts: Spread walnuts on a baking sheet and bake at 350 degrees F, stirring once, until fragrant, 7 to 9 minutes. Let the nuts cool on a plate.
Please note that some ingredients and brands may not be available in every store.