Step 1: Preheat a toaster oven or a regular oven to 375°F. Open the English Muffins and toast the English Muffin halves for about 3 minutes until very lightly browned. Remove the English Muffins and leave the oven on.
Step 2: Mix the tuna, mayonnaise, onion, celery, mustard, in a medium-sized bowl. Add the salt and pepper and blend well.
Step 3: Divide the tuna over the 4 English Muffin halves. Top each with 1/4 cup of shredded cheddar cheese and return to the oven for about 5 minutes until the cheese has melted. Serve warm.