Heat oven to 350°F. In medium bowl, combine tuna, celery, onion, lemon peel and mayonnaise; mix well. Spread about 3 tablespoons tuna mixture on each English muffin half. Top each with tomato slice; sprinkle with cheese. Place on ungreased cookie sheet.
Bake at 350°F. for 8 to 10 minutes or until cheese is melted and sandwiches are thoroughly heated.