A minha mãe diz que o salmão com espinafres fica leve e muito bom.

In European Portuguese, it is very common — and usually the most natural choice — to use the definite article before a possessive: a minha mãe, o meu pai, a minha casa.

So a minha mãe is the normal Portugal Portuguese way to say my mother.

If you say just minha mãe, it may sound less natural in Portugal, more literary, or dependent on a special context. In Brazilian Portuguese, dropping the article is much more common.

Diz is the 3rd person singular of the verb dizer (to say / to tell in some contexts) in the present indicative.

A few present-tense forms are:

• eu digo — I say
• tu dizes — you say
• ele/ela/você diz — he/she/you say

Since the subject is a minha mãe, the correct form is diz.

Here que means that and introduces the clause that follows:

• A minha mãe diz que... = My mother says that...

In English, that is often omitted:

• My mother says (that) salmon with spinach...

But in Portuguese, que is normally required in this structure. So A minha mãe diz que... is the natural form.

Portuguese uses definite articles more often than English, especially with nouns referring to foods, ingredients, or things being discussed in a general but concrete way.

So o salmão is very natural here.

Using just salmão is possible in some styles, especially in recipe titles, headings, or very compressed speech, but in a full sentence o salmão sounds more normal.

In Portugal, spinach is very often referred to as espinafres, in the plural, especially in cooking and everyday speech.

So:

• espinafres = spinach

The singular espinafre exists, but espinafres is extremely common and often sounds more natural in this kind of sentence.

Here fica comes from ficar, and it means something like:

• turns out
• ends up being
• comes out

With food, ficar is very common when talking about the result of a recipe or combination of ingredients:

• Fica bom = It turns out good / It comes out good
• Fica leve = It turns out light

If you used é, that would sound more like a general or inherent description:

• O salmão com espinafres é leve = Salmon with spinach is light

That is possible, but fica is especially natural when someone is talking about how a dish turns out.

Because leve and bom are adjectives describing o salmão com espinafres.

After verbs like ser, estar, and ficar, Portuguese usually uses an adjective to describe the subject:

• fica bom
• fica leve
• fica ótimo

Bem is an adverb, not an adjective. So muito bem would not fit this meaning.

Compare:

• O prato fica muito bom = The dish turns out very good
• Ela cozinha muito bem = She cooks very well

Because the adjective agrees with the noun it describes, and the head noun here is o salmão, which is masculine singular.

So:

• o salmão ... fica bom
• a sopa ... fica boa

Notice that leve does not show a masculine/feminine difference in the singular:

• o salmão fica leve
• a sopa fica leve

But in the plural it becomes leves.

Because the subject is the whole noun phrase o salmão com espinafres, and the main noun in that phrase is salmão, which is singular.

The phrase com espinafres is just an added complement meaning with spinach. It does not control the verb agreement.

So the structure is basically:

• [o salmão] = head noun, singular
• [com espinafres] = extra information

That is why the verb is fica, not ficam.

Here leve means light in the sense of food that is not too heavy, rich, greasy, or hard to digest.

So it does not mean physically light in weight. It means something more like:

• light to eat
• not too heavy
• easy on the stomach

That is a very common way to describe food in Portuguese.

The given order is the most neutral and natural:

• A minha mãe diz que o salmão com espinafres fica leve e muito bom.

Portuguese does allow some variation, but changes in order usually affect emphasis.

For example:

• A minha mãe diz que o salmão com espinafres fica muito bom e leve.

This is still understandable, but leve e muito bom sounds more natural because the speaker first gives one quality and then strengthens the overall positive judgment.

So the original sentence is a very normal, idiomatic word order.