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.
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.
Divide yogurt between 4 dessert bowls.