Ce dont j’ai envie ce soir, c’est d’une soupe de lentilles avec des petits pois.

Because the expression is avoir envie de quelque chose = to feel like / to want something.

So the missing element after envie is introduced by de:

• J’ai envie d’une soupe.

When French uses a relative pronoun to stand for de + thing, it uses dont:

• Ce dont j’ai envie...

So:

• ce que j’ai envie is not standard here
• ce dont j’ai envie is correct, because dont replaces de quoi / de quelque chose

A good shortcut is:

• que replaces a direct object
• dont often replaces de + noun

This is an emphasis structure.

It is similar to English:

• What I feel like tonight is lentil soup with peas.

The sentence is split into two parts:

1. Ce dont j’ai envie ce soir
   = the thing I feel like tonight

2. c’est d’une soupe de lentilles avec des petits pois
   = is lentil soup with peas

French often uses this kind of structure to highlight or frame the answer before giving it. It sounds a bit more deliberate or expressive than the simpler version:

• Ce soir, j’ai envie d’une soupe de lentilles avec des petits pois.

So the original is not wrong or strange; it is just more emphatic.

Yes, but the two ce forms are doing different jobs in the structure.

• The first Ce in Ce dont j’ai envie... means that which / what
• The second c’ in c’est is part of the fixed expression c’est, which is used to identify or present something

So the sentence follows a very common French pattern:

• Ce que je veux, c’est...
• Ce qui m’intéresse, c’est...
• Ce dont j’ai besoin, c’est...

It may feel repetitive to an English speaker, but in French it is completely normal.

Because the sentence is still tied to the expression avoir envie de.

The key idea is not simply it is a soup, but rather the thing I feel like is: a soup. Since avoir envie requires de, that de is kept in the second half:

• J’ai envie d’une soupe
• Ce dont j’ai envie, c’est d’une soupe

Compare:

• Ce que je veux, c’est une soupe
  because vouloir takes a direct object

but

• Ce dont j’ai envie, c’est d’une soupe
  because avoir envie de takes de

So the de is there because of the verb expression, not because c’est normally needs de.

Not exactly.

• avoir envie de = to feel like, to be in the mood for
• vouloir = to want

With food, avoir envie de is especially natural, because it often suggests craving, appetite, or preference at that moment.

So:

• J’ai envie d’une soupe
  = I feel like having a soup / I’m in the mood for a soup

• Je veux une soupe
  = I want a soup

The second one is not wrong, but it can sound more direct or stronger. In this context, avoir envie de is the more idiomatic choice.

Because de lentilles describes what kind of soup it is, or what it is made from.

In French, when naming dishes, drinks, or mixtures, de + ingredient is very common:

• une soupe de lentilles
• une salade de tomates
• un jus d’orange
• une purée de pommes de terre

Here, lentilles is not referring to some specific lentils already known in the conversation. It is just naming the ingredient/type. That is why French uses de lentilles, not des lentilles.

This is a very good question.

There is a common rule that des often becomes de before a plural adjective:

• des grands arbres → often de grands arbres

But petits pois is a fixed expression meaning peas or green peas. In practice, French treats petits pois almost like a set noun, not just any noun with an ordinary adjective added on the spot.

So:

• des petits pois is the normal form

Here petits is really part of the standard name of the vegetable, so the usual des → de tendency does not apply in the same way.

It means peas, especially green peas / garden peas.

Even though it literally looks like small peas, French speakers usually understand petits pois as the normal food word for peas. So in many contexts, it behaves like a fixed vocabulary item rather than a fully literal adjective + noun combination.

That is why learners should usually memorize it as one unit:

• des petits pois

Because it naturally modifies j’ai envie: it tells you when the speaker feels like this.

So:

• Ce dont j’ai envie ce soir
  = what I feel like tonight

French time expressions are fairly flexible. You could also say:

• Ce soir, j’ai envie d’une soupe de lentilles avec des petits pois.
• Ce dont j’ai envie, ce soir, c’est d’une soupe de lentilles avec des petits pois.

The original placement is perfectly natural and keeps the emphasis on what I feel like tonight.

It is natural, but it is more emphatic than the most neutral everyday wording.

A more straightforward sentence would be:

• Ce soir, j’ai envie d’une soupe de lentilles avec des petits pois.

The original version:

• Ce dont j’ai envie ce soir, c’est d’une soupe de lentilles avec des petits pois.

sounds like the speaker is deliberately highlighting the answer, almost as if responding to an unspoken question such as What do I feel like tonight?

So yes, it is good French; it is just a little more structured and expressive than the simplest option.