Combine the mayonnaise and lemon juice in a medium-size bowl. Season with salt and pepper to taste. Toast the pecans in a toaster oven for 1 minute, or until brown to bring out their flavor. Be careful-they burn easily. Toss the celery, apple and pecans in the mayonnaise mixture. Place lettuce leaves on 4 plates and spoon the salad onto the leaves to serve.
Open-Faced Roast Beef Sandwich:
Spread the bread with mustard. Divide the roast beef between each slice of bread. Top with the tomato slices. Serve any extra tomato slices on the side.