Wash the celery stalks under cool running water. Trim both ends and remove any tough strings. Cut each stalk into short sticks. Toss them gently with lemon juice, salt and black pepper.

Add mayonnaise and Dijon mustard to a small bowl. Stir until smooth and creamy. Taste the mixture and adjust the mustard if needed. This simple spread gives the sandwich a bright and savory flavor.

Spread a thin layer of butter over one side of each bread slice. Heat a clean skillet over medium heat. Toast the bread lightly until the edges become golden and the surface feels crisp.

Place the sliced roast beef in the warm skillet. Heat it gently for one to two minutes. Avoid cooking it too long. Overheating can make the beef dry and less tender.

Spread the mayonnaise mixture over the toasted bread. Add lettuce, tomato, and red onion. Place warm roast beef over the vegetables. Add pickle slices if you like extra tangy flavor.

Top each sandwich with another slice of bread. Press gently without flattening the filling. Cut each sandwich in half. Serve immediately with the prepared celery sticks for a fresh and crunchy side.