Unser Abendessen besteht aus Suppe und Brot.

Because Abendessen is a neuter singular noun in the nominative case.

• das Abendessen = dinner, evening meal
• With a neuter singular noun in the nominative, unser stays unser

So:

• unser Abendessen = our dinner
• unsere Suppe = our soup, because Suppe is feminine

German possessive words change their endings depending on gender, number, and case.

Because the subject is Unser Abendessen, which is third-person singular.

The verb is bestehen aus = to consist of.

Present tense of bestehen:

• ich bestehe
• du bestehst
• er/sie/es besteht
• wir bestehen
• ihr besteht
• sie/Sie bestehen

Since Abendessen is singular, you need:

• Unser Abendessen besteht ...

bestehen aus means to consist of or to be made up of.

So:

• Unser Abendessen besteht aus Suppe und Brot. = Our dinner consists of soup and bread.

This is a very common expression when talking about the parts or contents of something:

• Der Salat besteht aus Tomaten und Gurken.
• Das Team besteht aus zehn Leuten.

Be careful: bestehen can have other meanings too, depending on context:

• eine Prüfung bestehen = to pass an exam
• aus etwas bestehen = to consist of something

Because the verb bestehen normally goes with aus when it means to consist of.

So the pattern is:

• etwas besteht aus + dative

Examples:

• Der Kuchen besteht aus Mehl, Zucker und Eiern.
• Das Menü besteht aus drei Gängen.

You usually cannot replace aus with another preposition here.

Aus always takes the dative case.

So in:

• Unser Abendessen besteht aus Suppe und Brot.

both Suppe and Brot are understood in the dative.

Why does that not look different here?

Because there is no article, and many bare nouns do not visibly change form in the dative singular.

Compare with articles:

• aus der Suppe
• aus dem Brot
• aus den Zutaten (plural)

So the rule is still there: aus + dative.

Because German often leaves out articles when talking about food in a general or substance-like way.

So:

• aus Suppe und Brot sounds natural = consisting of soup and bread

This is similar to English, where you might also say soup and bread without a/the.

If you add articles, the meaning becomes more specific:

• aus der Suppe und dem Brot = out of/from the soup and the bread, or referring to very specific items in a context where the articles matter

In this sentence, the article-less version is the normal one.

Because German very often forms compound nouns by joining words together.

• Abend = evening
• Essen = food / meal / eating

Together:

• das Abendessen = dinner, evening meal

This is extremely common in German:

• Frühstück = breakfast
• Mittagessen = lunch
• Abendessen = dinner

English often writes these as separate words, but German usually combines them into one noun.

Because all nouns are capitalized in German.

So:

• unser = not capitalized, because it is a possessive determiner
• Abendessen = capitalized, because it is a noun
• Suppe, Brot = capitalized, because they are nouns

This is one of the most noticeable spelling rules in German.

Not usually, at least not if you want the natural meaning consists of.

• Unser Abendessen besteht aus Suppe und Brot = natural and idiomatic
• Unser Abendessen ist Suppe und Brot = understandable, but less natural in this context

German prefers besteht aus when naming the components of something.

The base form is unser-, the possessive word meaning our.

Like other German possessive determiners, it behaves a lot like an ein-word and changes its ending depending on the noun.

Examples:

• unser Abendessen = our dinner
• unsere Suppe = our soup
• unser Brot = our bread
• unsere Mahlzeiten = our meals

So unser is not a completely fixed form; it changes.

Yes. German commonly joins nouns with und exactly like English uses and.

So:

• Suppe und Brot = soup and bread

This kind of simple list is very normal, especially with food:

• Käse und Wein
• Fisch und Reis
• Kartoffeln und Gemüse

Nothing unusual is happening there; it is just a straightforward coordination of two nouns.

You would change the possessive and/or the noun, but the structure stays the same.

Examples:

• Mein Abendessen besteht aus Suppe und Brot.
  = My dinner consists of soup and bread.

• Unser Mittagessen besteht aus Suppe und Brot.
  = Our lunch consists of soup and bread.

The pattern is still:

• [possessive] + [meal noun] + besteht aus + [foods]