In the beginning, the materials are a tomato can (about 300g), olive oil, one onion, one paprikabell pepper, garlic (three peace), and a zucchini.

In the US, "paprika" is a dried and ground up red pepper. It is in a jar, like a spice. A "bell pepper" is a raw, fresh green, red, or orange pepper that is typically cut up into ratatouille. It is not spicy, more sweet. I think maybe you mean bell pepper? Here is a picture of a bell pepper: https://en.wikipedia.org/wiki/Bell_pepper

After a while, add the chopped paprikabell pepper too.

Then, put in sliced the zucchini and mushrooms.

I have to make dinner for myself because I live alone and restaurants in the university are nothe university dining halls aren't open due to COVID-19.

"Aren't" sounds more natural than "are not." I think you mean dining halls, which are the places where most students eat. Restaurants are more formal and expensive.

SoFortunately, I have found an almost perfect nutrition-ally balanced dish; it is R: ratatouille.

"Ratatouille" doesn't get capitalized. Your use of the semicolon was correct, but the colon seems more natural to me in this context. Technically, it's not grammatically correct to start a sentence with "so."

Ratatouille is a French domestic dish which isish that's a little like tomato soup with some vegetables.

I put in a contraction ("that's") because I think it better matches the informal tone of your writing. Just "French dish," no need to include the word "domestic."

Re: the difference between "that" and "which"...
https://www.grammarly.com/blog/which-vs-that/

I will show how to cook it in a brief process'll briefly summarize how to cook this dish.

In the beginning, the materials are aYou'll need the following ingredients: a can of tomato cansauce (about 300g), olive oil, one onion, one paprika, three garlic (three peace)cloves, and a zucchini.

Paprika is uncountable. "Clove" is the word for one of those little sections of garlic.

The first step is cuttingto cut the garlic and roast themit in a pot with olive oil in a pot.

IfOnce the garlic becomes crispy, add in the chopped onion.

"If the garlic becomes crispy" means the garlic might become crispy, but if it doesn't, that's okay. I think the goal here is for the garlic to become crispy – otherwise, the ratatouille won't turn out well – so "once" is a better word to use.

After a while, add chopped paprika, too.

I could be wrong, but I don't think paprika can be chopped? Because it's a spice, right?

In the endFinally, stew the vegetables with tomato on low heatsauce over a low flame.

Don't forget to adjust the taste by adding salt, pepper, and consommeé.

I think that basil goes well with this recipe.