1. Heat oven to 350°F.
2. Combine spinach, salt, onion, butter, eggs, and milk. Blend well.
3. Turn mixture into 6 greased custard cups.
4. Place custard cups in a pan of hot water. Bake 30 minutes.
5. Heat tomato sauce in a small saucepan.
6. Unmold spinach timbales and serve with hot tomato sauce.