Heute koche ich Brei aus Haferflocken und Banane für das Baby.

Because German main clauses usually follow the V2 rule: the finite verb must be in second position.

In this sentence, Heute is in the first position, so the verb koche has to come second:

• Heute | koche | ich | ...

If you started with the subject instead, you would get:

• Ich koche heute Brei ...

Both are correct. The original sentence simply puts Heute first to set the time frame.

Putting Heute first gives the sentence a natural time-first focus: Today, I’m cooking...

German often places the time expression first when it is the topic of the sentence. It does not dramatically change the meaning, but it slightly highlights today.

Compare:

• Heute koche ich Brei ... = emphasis on today
• Ich koche heute Brei ... = emphasis is a bit more neutral

In German, food words are often used without an article when speaking in a general or uncountable way.

So ich koche Brei means I’m cooking porridge/mash as a substance, not necessarily a specific porridge.

You could also say:

• Ich koche einen Brei.

That sounds more like I’m cooking a porridge / a serving of porridge / a particular kind of mash.

In this sentence, the version without an article sounds very natural.

Brei is masculine: der Brei.

Its accusative form with an article would be:

• der Brei → einen Brei

That is why, if you added an article, you would say:

• Ich koche einen Brei.

Here aus means made from / out of.

So:

• Brei aus Haferflocken und Banane = porridge made from oats and banana

This is a very common pattern in German:

• Suppe aus Gemüse
• Saft aus Orangen
• Brot aus Vollkornmehl

Aus always takes the dative case.

That means if the nouns had articles, you would see dative forms, for example:

• aus den Haferflocken
• aus einer Banane

In your sentence, the articles are omitted, so the case is not very visible on the nouns themselves.

This is a kind of recipe-style or ingredient-style wording.

German often leaves out articles when listing ingredients, especially in informal cooking contexts:

• mit Apfel
• aus Reis und Gemüse
• mit Banane

So aus Haferflocken und Banane sounds like made from oats and banana.

If you wanted to be more explicit, you could say:

• aus Haferflocken und einer Banane

That would clearly mean using one banana.

Because the sentence most naturally suggests banana as an ingredient, or possibly one banana.

German often uses ingredient nouns in this bare form in cooking language. So Banane here works a bit like an ingredient label.

Depending on the situation, you could also hear:

• aus Haferflocken und Bananen = made from oats and bananas, in general or several bananas
• aus Haferflocken und einer Banane = made from oats and one banana

Haferflocken means oat flakes or rolled oats.

It is a very common German word for the oats you use in porridge, muesli, and baking.

So this is more specific than just Hafer (oats as a plant/grain). In everyday cooking, Haferflocken is the normal word.

Because für always takes the accusative case.

So:

• für das Baby = for the baby

If it were a preposition that required dative, then you might see dem Baby, but not after für.

Yes. Fürs is the contraction of für das.

So these are both correct:

• für das Baby
• fürs Baby

Fürs Baby is very common in everyday speech.
Für das Baby can sound a little fuller or slightly more careful/emphatic.

German word order is flexible, but some orders sound more natural than others.

In this sentence, the speaker first says what they are cooking:

• Brei aus Haferflocken und Banane

and then adds for whom:

• für das Baby

That gives a smooth, natural flow.

You could also say:

• Heute koche ich für das Baby Brei aus Haferflocken und Banane.

That is also grammatical, but the original version sounds very natural and clear.

Yes. Baby is usually neuter in German:

• das Baby

This is normal, even though it refers to a person. German grammatical gender does not always match biological sex.

Yes, absolutely.

Both are correct:

• Heute koche ich Brei aus Haferflocken und Banane für das Baby.
• Ich koche heute Brei aus Haferflocken und Banane für das Baby.

The difference is mainly focus:

• Heute koche ich... = highlights today
• Ich koche heute... = more neutral subject-first order

Yes, it is natural German.

A few slightly different versions are also possible depending on style:

• Heute koche ich Brei aus Haferflocken und Banane fürs Baby.
• Heute mache ich Haferflocken-Bananen-Brei fürs Baby.
• Heute koche ich dem Baby Brei aus Haferflocken und Banane.

But your original sentence is perfectly understandable and natural.