Dale la vuelta a la merluza cuando esté crujiente por un lado.

Because the basic tú command of dar is da, and then Spanish adds the pronoun le: da + le = dale.

In this very common expression, darle la vuelta a algo, the le refers to the thing you are turning, here la merluza. So dale la vuelta a la merluza is a very natural way to say turn the hake over.

You may also hear da la vuelta a la merluza, but dale la vuelta is especially common in everyday cooking Spanish.

Here dar la vuelta means to turn over or to flip.

So in cooking, dale la vuelta a la merluza means you should turn the fish so the other side cooks.

Literally, it is something like give it the turn, but you should learn it as a fixed expression meaning flip/turn over.

That a is part of the expression dar(le) la vuelta a algo.

So:

• dar la vuelta a la merluza
• darle la vuelta a la merluza

both use a before the thing being turned.

It is not the same as the personal a used with people. It is just how this structure works.

Because after cuando, Spanish normally uses the subjunctive when talking about something that has not happened yet.

Here, the fish is not crispy yet at the moment of speaking. The instruction means:

• turn it over when it becomes crispy on one side

That is why Spanish uses cuando esté crujiente.

A very useful rule is:

• cuando + future/expected action → usually subjunctive
• cuando + habitual or already known action → often indicative

Because esté is the present subjunctive form of estar, and that form is spelled with an accent:

• esté
• estés
• esté
• estemos
• estéis
• estén

So the accent is simply part of the correct spelling of that verb form.

Because crujiente is an adjective that usually has the same form for masculine and feminine.

So you get:

• pan crujiente
• merluza crujiente

It only changes for plural:

• crujiente
• crujientes

In cooking, crujiente means crispy or crisp.

Here it does not mean the whole fish is crunchy all the way through. It means the outside has become nicely crisp on that side.

So esté crujiente por un lado means the fish has become crispy on one side.

Here por un lado means on one side.

In this cooking context, it suggests:

• one side is already crispy
• the other side still needs cooking

So the instruction is telling you to flip the fish once the first side is ready.

It is informal singular, used with tú.

Other versions would be:

• dale la vuelta = informal singular
• dele la vuelta = formal singular
• dadle la vuelta = informal plural in Spain (vosotros)

So the sentence is addressing one person in an informal way.

Because Spanish usually leaves out subject pronouns when the verb already makes the subject clear.

Dale already shows that this is a tú command, so adding tú would normally be unnecessary.

Spanish often sounds more natural without the subject pronoun unless you want special emphasis.

You could, and it would be understood, but darle la vuelta is much more natural for flipping food while cooking.

Girar is more general, like turn or rotate.
For food in a pan, Spanish very commonly uses:

• dar la vuelta
• darle la vuelta

So dale la vuelta a la merluza is the most idiomatic everyday choice here.