Toast muffins, and set aside.
Pour beaten egg in a 6-ounce custard cup.
Cover with heavy-duty plastic wrap, and microwave at MEDIUM (50% power) for 1 to 1 1/2 minutes or until egg is set.
Let stand 3 minutes.
Mash egg with a fork until crumbly.
Combine egg, tuna, mayonnaise, cheese, green onions, and mustard, stirring well.
Spread over toasted muffins; microwave at HIGH for 1/2 to 2 minutes or until heated.
Sprinkle with paprika, and garnish with cherry tomato halves and parsley sprigs, if desired.