In a large mixing bowl, combine the mayonnaise, Dijon mustard, lemon juice, and Old Bay seasoning. Mix well until smooth.
Gently fold in the lump crab meat, ensuring not to break it apart too much.
Add the chopped celery, red bell pepper, and green onions to the crab mixture. Stir gently to combine.
Season with salt and pepper to taste. Adjust the seasoning as needed.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes to allow the flavors to meld.
Serve the crab salad on a bed of lettuce leaves or as a filling for sandwiches or wraps.