Wash potatoes and cut into cubes. Leave the skin on, if you like. In a covered saucepan, cook potatoes in enough boiling water to cover for 20 to 25 minutes or just until tender. Drain well.
In a large bowl, combine mayonnaise dressing, celery, green pepper, green onions, mustard, vinegar, salt, and black pepper.
Add potatoes and egg. Toss lightly to mix. Cover and chill in the refrigerator for at least 6 hours or up to 24 hours. If you'd like a creamier salad, stir in the milk before serving.
Serve garnished with celery leaves and sprinkled with paprika, if you like. Makes 6 side-dish servings.
If you're watching the cholesterol in your diet, you can leave out the egg. For variety, try substituting a different sweet pepper -- yellow, brown, purple, red, or orange -- for the green one.