По мере того как суп с фасолью и чечевицей остывал, его вкус становился ярче.

It means as, in proportion as, or the more... the more... in a gradual sense.

It is used when two processes develop alongside each other over time. In this sentence, one thing is happening gradually — the soup is cooling — and another thing is changing gradually at the same time — the taste is becoming more vivid.

So по мере того как is stronger and more specific than a simple when.

In this sentence, по мере того как is a fixed conjunction meaning as / as... gradually.

So you should learn по мере того как as one whole unit.

There is also another pattern:

• по мере + noun
• for example: по мере остывания супа = as the soup cooled / during the cooling of the soup

But when a full clause follows, Russian commonly uses по мере того как.

Because the sentence describes a process, not just a finished result.

• остывал = was cooling / cooled gradually
• становился = was becoming

Both changes happened little by little, so the imperfective is natural.

If you used perfective verbs, the meaning would shift toward a completed result:

• Когда суп остыл, его вкус стал ярче.
• When the soup cooled down, its taste became more vivid.

That version is about the result after cooling, not the gradual change during cooling.

Because the subject is суп, and суп is masculine singular.

In the past tense, Russian verbs agree with the subject in gender and number:

• суп остывал — masculine
• каша остывала — feminine
• молоко остывало — neuter
• блюда остывали — plural

The same thing happens with становился, which agrees with вкус. Since вкус is also masculine singular, you get становился.

This is the instrumental case.

The preposition с meaning with normally takes the instrumental:

• с фасолью = with beans
• с чечевицей = with lentils

Base forms:

• фасоль
• чечевица

Instrumental forms:

• фасолью
• чечевицей

So суп с фасолью и чечевицей literally means soup with beans and lentils.

Yes, very natural.

Russian often describes dishes with X с Y:

• салат с курицей — salad with chicken
• рис с овощами — rice with vegetables
• суп с грибами — soup with mushrooms

So суп с фасолью и чечевицей is a normal, idiomatic way to describe the contents of the soup.

You might also see adjective-based names like фасолевый суп, but with two ingredients, суп с фасолью и чечевицей is especially straightforward.

Here его means its, referring to суп.

So:

• его вкус = its taste

Russian его can mean his or its. Context tells you which one is meant.

The sentence could also have said вкус супа, but его вкус sounds smoother and avoids repeating суп.

Ярче is the comparative form of яркий:

• яркий — vivid, bright
• ярче — more vivid, brighter

After verbs like становиться, Russian very often uses the comparative to show change:

• становиться лучше — become better
• становиться хуже — become worse
• становиться сильнее — become stronger
• становиться ярче — become more vivid

So его вкус становился ярче means its taste was becoming more vivid / more pronounced.

Yes, that is possible.

• становился ярче — more natural, more compact, very idiomatic
• становился более ярким — also correct, but slightly heavier in style

In everyday Russian, the simple comparative ярче is usually the better choice here.

Yes.

Although яркий literally means bright or vivid, Russian often uses it metaphorically for things like:

• taste
• aroma
• emotions
• language
• impressions

So яркий вкус means something like:

• vivid taste
• pronounced taste
• intense flavor

It does not refer to visual brightness here.

Because the sentence has a subordinate clause followed by a main clause.

Structure:

• По мере того как суп с фасолью и чечевицей остывал — subordinate clause
• его вкус становился ярче — main clause

Russian puts a comma between them.

Not without changing the nuance.

• по мере того как emphasizes a gradual, parallel development
• когда usually just means when

Compare:

• По мере того как суп остывал, его вкус становился ярче.

  • The cooling and the flavor change happened progressively together.

• Когда суп остыл, его вкус стал ярче.

  • First the soup cooled; then we describe the result.

So когда is possible in a different sentence, but it does not express quite the same idea.

Yes. Russian word order is flexible.

For example, you could also say:

• Его вкус становился ярче по мере того как суп с фасолью и чечевицей остывал.

That is grammatical. The original version sounds natural because it presents the background process first and then the result of that process.

So the word order here is not random, but it is not the only possible one either.