Warm and comforting, grilled cheese sandwiches are always a big hit with everyone in the family. Look for light tuna such as skipjack since it is a great source of protein and usually lower priced than white tuna.
In a small bowl, mix together tuna, celery, mayonnaise, lemon juice and pepper.
Spread bread with butter on one side of each slice. Divide tuna mixture on two of the unbuttered sides of the bread. Top tuna with cheese slice and other slice of bread, buttered side up.
Place sandwiches in large nonstick skillet over medium heat and cook for about 2 minutes per side until golden brown.
Remove from skillet and gently lift bread slice and tuck baby spinach and tomato inside the sandwich to serve.