El puré de patata con un poco de perejil queda muy bien con la merluza.

Both are possible.

In Spain, names of dishes often use the singular after de to mean the ingredient in a general sense, so puré de patata sounds natural. You also hear puré de patatas, especially when people are thinking of the actual potatoes used.

This is similar to other food names in Spain, such as:

• tortilla de patata
• crema de calabaza
• zumo de naranja

So de patata here does not mean only one potato. It is just a common way to name the dish.

Here quedar has a different, very common meaning: to suit, to go well with, or to work well with.

So in food contexts:

• El puré queda muy bien con la merluza = the mashed potato goes very well with hake.

You can also hear quedar bien con for clothes, colors, decoration, etc.:

• Ese color te queda bien = That color suits you.
• El vino blanco queda bien con pescado = White wine goes well with fish.

So this is an idiomatic use of quedar.

Because the subject is singular: El puré.

The full subject is:

• El puré de patata con un poco de perejil

The main noun there is puré, which is singular, so the verb must also be singular:

• El puré ... queda

The phrases de patata and con un poco de perejil just add information; they do not change the subject from singular to plural.

Because bien is an adverb here, and it modifies the verb queda.

• quedar bien = to go well / to suit
• quedar muy bien = to go very well

You would not normally say queda muy bueno in this context.

Compare:

• Está muy bueno = It tastes very good.
• Queda muy bien con la merluza = It goes very well with hake.

So bueno talks more about the quality or taste of something itself, while bien in quedar bien con talks about how well it matches something else.

In Spanish, articles are often used with foods and dishes in a more general or meal-related way than in English.

So con la merluza sounds natural when referring to hake as the fish dish being served.

Spanish often keeps the article where English would drop it:

• Me gusta el café
• La pasta combina bien con...
• El arroz va bien con el pollo

Here la merluza can mean hake in general, or the hake dish in this meal/context.

Yes, absolutely.

These are all natural:

• queda muy bien con la merluza
• va muy bien con la merluza
• combina muy bien con la merluza

There are slight differences in feel:

• quedar bien con often sounds a bit like to suit or to work nicely with
• ir bien con is very common and slightly more everyday
• combinar bien con sounds a little more explicit or descriptive

So the original sentence is natural, but it is not the only option.

Because perejil is being used as an uncountable ingredient here.

After un poco de, Spanish normally uses de + noun without the article when talking about some amount of a substance or ingredient:

• un poco de sal
• un poco de agua
• un poco de perejil

You would only use del if you meant a specific parsley already identified in the conversation, but that would be unusual here.

So un poco de perejil is the normal form.

It adds extra information about the mashed potato.

The structure is:

• El puré de patata = the mashed potato
• con un poco de perejil = with a little parsley
• queda muy bien con la merluza = goes very well with hake

So con un poco de perejil is describing the purée, not the hake.

In other words, it means:

• the mashed potato, when it has a little parsley in it, goes very well with hake

Yes, puré is a masculine noun: el puré.

There is no simple rule based only on the ending -é. Noun gender in Spanish must often just be learned with the word.

Some nouns ending in -é are masculine, for example:

• el puré
• el café
• el té
• el sofá

So it is best to learn it as a full unit:

• el puré

The accent mark shows the stress.

Without the written accent, pure would follow normal spelling rules and be pronounced differently. The correct word is:

• puré

with the stress on the last syllable.

So the accent is not optional; it is part of the standard spelling.

Merluza is standard Spanish and is very common in Spain. It means hake, a fish that is especially common in Spanish cooking.

A learner of Spanish from England or the US may notice that this fish is less culturally central in English-speaking contexts, but in Spain merluza is an everyday food word.

So this sentence sounds very natural in a Spain food context.

That word order is the most natural here because the sentence is building toward the pairing.

Spanish often places the matching element after quedar bien con:

• El arroz queda bien con el pollo
• Esta salsa queda bien con la carne
• El puré queda muy bien con la merluza

You could rearrange things in some contexts, but the original order is smooth and standard.

Not exactly.

• un poco de perejil = a little parsley
• poco perejil is possible in some structures, but it does not fit as naturally here

With nouns, Spanish very often uses:

• un poco de + noun

So con un poco de perejil is the normal and idiomatic choice.

It also sounds slightly softer and more natural than a bare poco in this sentence.

Yes. El puré de patatas con un poco de perejil queda muy bien con la merluza is also perfectly natural in Spain.

A learner should understand that both versions exist:

• puré de patata
• puré de patatas

The singular can sound more like the name of the dish or ingredient type; the plural can sound more literal. In real life, both are used.