غذای آن رستوران همیشه خوب است.

Because this is the ezafe construction.

• غذا = food
• غذای آن رستوران = the food of / at that restaurant

The ی here represents the ezafe link after a word ending in a vowel. It is pronounced roughly as -ye:

• غذای آن رستوران
  ghazâ-ye ân resturân

So the ی does not make the word plural or change its basic meaning. It simply links غذا to what comes after it.

Yes, literally it is very close to the food of that restaurant.

But in natural English, it is often better translated as:

• The food at that restaurant
• That restaurant's food

Persian often uses this noun-linking structure where English might prefer at or an apostrophe 's.

Persian does not have a definite article like English the.

So غذا can mean:

• food
• the food

The exact meaning depends on context.

In this sentence, English naturally uses the:
The food at that restaurant is always good.

But Persian does not need a separate word for the.

Because آن means that, and in Persian demonstratives come before the noun.

• آن رستوران = that restaurant
• این رستوران = this restaurant

So the order is similar to English here:

• that restaurant
• آن رستوران

In everyday spoken Persian, آن is often replaced by اون.

همیشه means always. It is an adverb of frequency.

In this sentence, it tells you how often the food is good:

• غذای آن رستوران همیشه خوب است.
• The food at that restaurant is always good.

Its position is very natural here: after the subject and before the predicate.

Because in Persian, when you have a sentence with to be, the predicate usually comes before the verb است.

So the structure is:

• subject + predicate + است

Here:

• غذای آن رستوران = subject
• همیشه خوب = predicate phrase
• است = is

So Persian says something like:

• The food of that restaurant always good is

That is normal Persian word order.

In formal written Persian, you normally keep است.

In spoken Persian, it is usually shortened rather than completely omitted:

• formal: غذای آن رستوران همیشه خوب است
• spoken: غذای اون رستوران همیشه خوبه

So learners should recognize:

• است = formal / written is
• ـه / ه = common spoken is

A very natural spoken version is:

• غذای اون رستوران همیشه خوبه.

Changes:

• آن → اون
• است → ـه
• pronunciation is more conversational overall

So:

• written/formal: غذای آن رستوران همیشه خوب است.
• spoken/everyday: غذای اون رستوران همیشه خوبه.

Both mean the same thing.

A careful pronunciation is:

• ghazâ-ye ân resturân hamishe xub ast

A more natural spoken pronunciation is closer to:

• ghazâ-ye un resturân hamishe xoobe

A few useful notes:

• غ sounds like a French or Arabic-style gh sound
• خ in خوب is the kh sound, like German Bach
• خوب is often transliterated xub or khub
• است is often pronounced ast in careful speech, but in conversation becomes -e

It is an adjective.

Here خوب means good, and it describes غذا:

• the food ... is good

It is not an adverb like well.

So:

• خوب = good
• همیشه = always

That means:

• همیشه خوب است = is always good

A little, yes, but the given sentence is the most neutral and natural.

Standard version:

• غذای آن رستوران همیشه خوب است.

You might also hear:

• همیشه غذای آن رستوران خوب است.

This puts more emphasis on always.

But for learners, the original order is a very good model:

• subject + همیشه + adjective + است