How to describe not eating oil and water?

The most common description of oil-free and water-free food is a four-character idiom, that is, "clear soup is not mixed with water."

Think about it, the soup in the bowl is clear. It's soup, but it's actually a bowl of water, and it's nothing, not much. Such a meal, where can you see a little oil and water! But this kind of food, in the evil old society, is a common practice of poor families.