Celery with Cream Cheese Mousse

This delicious and light snack will please the young . . . and young at heart

¼ C low-fat whipped cream cheese
¼ C fat-free plain yogurt
2 Tbsp scallions (green onions), rinsed and chopped
1 Tbsp lemon juice
½ tsp ground black pepper
6 celery sticks, rinsed, with ends cut off
1 Tbsp chopped walnuts