(In a casual tone) ¶
I mixed bonito flakes with steamed rice and seasoned it with soy sauce for lunch today.

I'm not sure you need to explain this is in a casual tone. It would be natural to write like this about a personal topic (compared to an essay for school). We do not have polite and casual speech in English the way Japanese has different forms of speech.

After many years, I am finally convinced that the amount of bonito used is very small, so if it's contaminated in any way it'll be ok.

I will not use it sporadicallyvery often.

I've just learned that it's eastern Japanese style cat meal, and in western area it's miso-soup over rice.

Does this mean in the east the cats eat "nekomanma" and in the west, the cats eat miso soup over rice?

Today's lLunch

(In a casual tone) ¶
For lunch today, I mixed bonito flakes with steamed rice and seasoned it with soy sauce for lunch today.

After many years, I am finally convinced that the amount of ADD SUBJECT HERE used is very small, so if it's contaminated in any way it'll be ok.

I am not sure what the subject here is. Is it the soy sauce you are talking about?

I will use it sporadically.

I am not sure what “it” is referring to in this sentence.

I've just learned that it's eEastern Japanese style cat meal, and in the western area of Japan it's miso-soup over rice.

You should capitalize words north/east/south/west when they are used with a proper noun. When you are talking about a general area, you do not need to capitalize. Ex: Eastern Japan vs east of Japan. Western Europe vs. the western part of Europe.

After many years, I am finally convinced that the amount I usually used is very small, so if it'seven if the flakes are contaminated in any way it'll will likely be ok.

Anyway, it's a bewell-loved mealdish in Japan from ... I don't know, but since when, but it is a from a long, long time ago.

I've just learned that it's eastern Japanese style cat meal, and inthis type of cat food is found in eastern Japan, while in the western area it's, miso-soup over rice is served instead.

After many years, I am finally convinced that the amount I used is very small, so if it's contaminated in any way it'll still be ok.

It wasn't wrong but this makes it clearer to me that you are referring to the flakes.

It's also famous for being fed to cats, andwhich we call it 'nekomanma'.

I've just learned that it's an eastern Japanese style cat meal, and in western area but in the west it's miso-soup over rice.

I don't really understand this part so my correction may be incorrect.