Place the broiler rack in the position nearest the heating element and turn on the broiler.

Dice the celery, and peel and finely chop the onion. Spray a 12-inch nonstick skillet with cooking oil spray and place it over medium heat. Add the celery and onion and cook until the onion is tender, about 2 minutes.

Drain the tuna and add it to the skillet along with the cottage cheese, mayonnaise, lemon juice, and garlic powder. Stir well to combine and break up the tuna chunks. Cook just long enough to warm the ingredients, 2 to 3 minutes, stirring frequently. Remove the pan from the heat and set aside.

Spray a 13 X 9-inch glass baking dish with cooking oil spray. Split the English muffins in half and place the halves, split side up, side by side in the baking dish. Spread the tuna mixture evenly over the muffins and sprinkle with the cheese. Broil until the cheese melts, 2 to 3 minutes. Serve at once.