Translation:The chef used to put butter in all the dishes.
The English simple past "put" would be translated into French with the passé composé "a mis". The imperfect is translated as "was putting" or "used to put".
The author of the sentence apparently intended "plat" not to mean the piece of tableware, but the food that is prepared and served. Calling menu items "plates" instead of "dishes" sounds a bit odd to me, but perhaps not to others.
This is a good example of the flexibility of this tense. "The chef used to put butter in all the dishes is correct, but also marked correct was, "The chef was putting butter in all the dishes." The first is a habit of the chef and the second is something he was doing in the past.
« Dans » means “in” and “on” would have been “sur”. It depends on the shape of the dish. I would use “in” for a bowl and “on” for a plate.
Adjectives which describe nouns must match the noun with endings for gender and number. So, “plats” is masculine plural and it requires “tous”. If you had a masculine singular noun, then you would use “tout”. Feminine singular takes “toute” and feminine plural takes “toutes.” How can you have “all” with a singular noun? Try a mass noun, for example water, food, time.