Classic Shepherd's Pie with Mashed Potato Topping

1 1/2 lbs yellow potatoes, pared and cubed
1 1/2 lbs ground beef
1 medium yellow onion, chopped
1 rib celery, chopped
1/2 green bell pepper, chopped
1 (15oz) can tomato sauce
1 (6oz) can tomato paste
2 cups cooked mixed vegetables
2 cups cooked green beans
1 teaspoon salt
1/8 teaspoon black pepper
Mashed Potatoes:
2/3 cup warm milk
1 egg
1/2 teaspoon salt
paprika for top