Linsen mit Brokkoli sind für mich ein gutes Mittagessen.

Because the grammatical subject is Linsen mit Brokkoli, and the head noun there is Linsen, which is plural.

So German makes the verb agree with Linsen, not with Mittagessen.

• Linsen ... sind ... = Lentils ... are ...
• Das Mittagessen ist gut. = The lunch is good.

Even though ein gutes Mittagessen is singular, it is the complement after sein, not the subject.

In German, Linse means lentil in the singular, and Linsen is the normal plural form.

When talking about lentils as food, German often uses the plural:

• Ich esse gern Linsen. = I like eating lentils.

So Linsen mit Brokkoli is a very natural way to mean lentils with broccoli.

Because the sentence is speaking about lentils in a general sense, not a specific set of lentils.

German often uses no article with plural nouns when talking about things in general:

• Hunde sind freundlich. = Dogs are friendly.
• Linsen sind gesund. = Lentils are healthy.

If you meant specific lentils, you could use an article:

• Die Linsen mit Brokkoli sind lecker. = The lentils with broccoli are tasty.

Because Brokkoli is often treated like a mass noun or ingredient name, especially in food descriptions.

So mit Brokkoli is like saying with broccoli in English.

That is very common in German:

• Reis mit Gemüse
• Nudeln mit Käse
• Linsen mit Brokkoli

If you were talking about specific broccoli, an article could appear in another context, but here it is not needed.

Mit always takes the dative case.

So in mit Brokkoli, Brokkoli is in the dative.

However, you do not see a special ending on Brokkoli here, because many nouns do not visibly change when there is no article.

You can see the dative more clearly with an article:

• mit dem Brokkoli
• mit einer Suppe

Für always takes the accusative case.

That is why German says:

• für mich = for me
• für dich = for you
• für ihn = for him

Here, mich is the accusative form of ich.

Here für mich means something like:

• for me
• to me
• in my opinion
• as far as I’m concerned

So the sentence is not claiming that lentils with broccoli are objectively a good lunch for everyone. It is presenting a personal view.

A natural paraphrase would be:

• Lentils with broccoli are a good lunch for me.
• For me, lentils with broccoli make a good lunch.

Because Mittagessen is a neuter noun, and after ein in the nominative singular neuter, the adjective takes -es.

So:

• ein gutes Mittagessen
• ein gutes Buch
• ein kaltes Getränk

This is standard adjective inflection after an indefinite article.

Because it is a noun, and all nouns are capitalized in German.

Mittagessen is one compound noun:

• Mittag = midday / noon
• Essen = food / meal / eating

Together, Mittagessen means lunch.

German very often combines words into one noun like this.

Here it is definitely a noun meaning lunch.

It is related historically to the verb essen, but in this sentence it functions as a normal noun:

• it is capitalized
• it comes with ein
• it has an adjective before it: gutes

So grammatically it behaves just like any other noun.

Yes. That is also correct.

German word order is flexible enough to move für mich to the front for emphasis:

• Linsen mit Brokkoli sind für mich ein gutes Mittagessen.
• Für mich sind Linsen mit Brokkoli ein gutes Mittagessen.

The basic meaning stays the same, but the second version emphasizes for me a little more.

Because mit suggests that Brokkoli is something accompanying the lentils.

• Linsen mit Brokkoli = lentils with broccoli

If you said Linsen und Brokkoli, that would simply mean lentils and broccoli, with both named more equally.

So the difference is small, but mit often sounds more like a dish description.

Yes, it is natural and correct.

It sounds like a normal personal statement about food. A native speaker might also say:

• Für mich sind Linsen mit Brokkoli ein gutes Mittagessen.
• Linsen mit Brokkoli sind ein gutes Mittagessen für mich.

But the original sentence is perfectly fine.