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, red bell pepper, celery, and green onions until evenly coated. Be careful not to break up the crab meat too much.
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.
To serve, place a generous handful of mixed greens on each plate and top with the crab salad. Garnish with lemon wedges.