My wife has developed, or become aware of, a gluten intolerance. That means no pasta, obviously; no breadcrumbs; and no flour-thickened sauces. The last constraint is good for stews, I think. But sometimes, in other recipes, you want an ingredient to bind things together.
Last night, I made some spinach with cream and cheese. Normally, I would use a bechamel sauce. This time, I put about 150ml of creme fraiche into a saucepan, and simmered it until thick. Meanwhile, I cooked a bagful of spinach, drained it, and squeezed out as much liquid as I could. I stirred about three heaped tbsps of Gruyere into the cream, and tipped in the spinach.
The sauce turned runny. Perfectly pleasant, but not what I had aimed for.