das Mus
purée
A thick, mushy mass made by cooking and puréeing fruit or vegetables.
Zum Nachtisch gibt es Pfannkuchen mit Apfelmus.
For dessert, there are pancakes with applesauce.

purée / mush
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purée
A thick, mushy mass made by cooking and puréeing fruit or vegetables.
Zum Nachtisch gibt es Pfannkuchen mit Apfelmus.
For dessert, there are pancakes with applesauce.

As a rule, 'Mus' (purée / mush) is an uncountable noun and is used without an article or with 'etwas' (some). People normally do not say 'einem Mus' or 'zwei Musen'.
Möchtest du noch etwas Mus zu deinen Kartoffelpuffern?
Would you like some more purée with your potato pancakes?
The word 'Mus' (purée) is often used as the second part of compound words to name the type. The most common form is 'Apfelmus' (applesauce).
Im Herbst kochen wir oft unser eigenes Pflaumenmus.
In autumn, we often make our own plum purée.
In the colloquial meaning, 'Mus' (mush) is almost always used with the preposition 'zu' (to / into). It vividly describes the result of destruction: something becomes 'zu Mus' (into mush).
Die Tomaten in der Tasche wurden leider zu Mus gequetscht.
Unfortunately, the tomatoes in the bag got squashed into mush.
porridge / mush
'der Brei' and 'das Mus' can both refer to a soft, thick mass, especially in food. 'Brei' is a more general word and fits well with foods made from grains, fruit, or vegetables, for example 'Babybrei' (baby food / baby purée) or 'Haferbrei' (oatmeal / porridge). 'Mus' is used more when fruit or vegetables have been cooked and puréed, for example 'Apfelmus' (applesauce). Both words can also be used for a crushed mass. They can be confused because the consistency is similar. As a practical rule: with puréed fruit or vegetables, 'Mus' often sounds more precise; with cooked soft food in general, 'Brei' is usually more appropriate.

Zum Frühstück gibt es Joghurt mit Apfelmus.
For breakfast, there is yogurt with applesauce.
Das Baby isst am Abend einen warmen Brei aus Hafer und Banane.
In the evening, the baby eats a warm porridge made from oats and banana.