What is the similarity? First of all, we are not familiar with a napper in cooking.

We are only familiar with napper as the French infinitive verb meaning to cover evenly with thick gravy or sauce, coming directly from the French word for tablecloth. In English, the infinitive to nap has the same meaning.

Concassé as a noun, refers to a chopped mixture, most often peeled tomatoes, and you will generally hear it referred to as a tomato concassé.

Concasser as a verb is the process of chopping or pounding any food, and, although it is often thought of as a coarse chop, it can also be quite fine. Parsley, chervil, tarragon, or other herbs can be concasséed with a few quick chops. The bones of meat, poultry, game, or fish can be concasséed with a cleaver for a stock.

Crushed ice that is used to line a serving dish can also be referred to as a concassé, although we have never heard it used that way in English.

It is possible that some people refer to the even covering of sauce or gravy as a napper, but we have not heard it. Thank heavens!