Solo quiero una rebanada de pan con queso.

Here solo means only / just.

In modern standard Spanish, it is normally written solo, without an accent. You may still see sólo in older materials or when someone wants to avoid ambiguity, but the usual current spelling is solo.

So Solo quiero... = I only want... / I just want...

Spanish often leaves out subject pronouns when they are not needed.

Quiero already tells you the subject is I, because it is the first person singular form of querer.

So:

• Quiero una rebanada... = I want a slice...
• Yo quiero una rebanada... = also correct, but yo adds emphasis, contrast, or clarity

You would use yo if you want to stress I, for example:

• Yo quiero una rebanada, no él.

Quiero is the present tense of querer, and it literally means I want.

It is completely grammatical and easy to understand, but depending on tone and situation, it can sound a bit direct in Spain, especially in a shop, café, or restaurant.

More polite or softer alternatives are:

• Quería una rebanada de pan con queso.
• Me gustaría una rebanada de pan con queso.
• Solo quería una rebanada de pan con queso.

So quiero is not wrong, but learners often want to know that native speakers may soften requests in real life.

Because pan by itself usually means bread in a general or uncountable sense, while una rebanada gives you one slice as a unit.

Spanish often needs a unit word when talking about part of something:

• una rebanada de pan = one slice of bread
• un trozo de pastel = a piece of cake
• un vaso de agua = a glass of water

If you just say quiero pan con queso, that means I want bread with cheese, but it does not specify one slice.

Here de means something like of in English: a slice of bread.

This is a very common Spanish structure:

• una taza de café
• una botella de agua
• un trozo de queso
• una rebanada de pan

So rebanada de pan literally works like slice of bread.

Rebanada means slice, especially for things like bread or cake.

For this sentence, it is a good choice because it clearly suggests a slice of bread.

Some related words are:

• rebanada = slice, especially of bread or cake
• trozo = piece, a more general word
• loncha = a thin slice, often of cheese, ham, or cold cuts
• rodaja = round slice, like a slice of tomato or lemon

So if you are talking about bread, rebanada is very natural.

Because con queso means with cheese, meaning the bread is served or eaten together with cheese.

• pan con queso = bread with cheese

If you say de queso, that usually changes the meaning. De queso often suggests that something is made of cheese, contains cheese as its main defining ingredient, or is a cheese type/flavor, depending on the noun.

Compare:

• pan con queso = bread with cheese
• bocadillo de queso = cheese sandwich
• pastel de queso = cheesecake

So in your sentence, con queso is the natural choice.

By itself, pan con queso is a bit broad. It simply means bread with cheese.

The exact image depends on context:

• bread with cheese on it
• bread served together with cheese
• bread and cheese as a simple snack

If you want to be more specific, Spanish can do that:

• pan con queso encima = bread with cheese on top
• un bocadillo de queso = a cheese sandwich
• una tostada con queso = toast with cheese

So the original sentence is understandable, but it is not extremely specific about the arrangement.

Yes, but the original order is the most neutral and natural:

• Solo quiero una rebanada de pan con queso.

Spanish word order is flexible, but moving solo can slightly change what is being emphasized.

For example:

• Solo quiero una rebanada de pan con queso.
  This usually means I only want one slice of bread with cheese or that’s all I want.

• Quiero solo una rebanada de pan con queso.
  Also possible, with a similar meaning, but a bit less neutral.

In general, putting solo before the verb is very common and natural.

Una is the feminine singular indefinite article, meaning a / one.

It matches rebanada, which is a feminine noun:

• la rebanada
• una rebanada

You cannot use un here because rebanada is not masculine.

So the agreement is:

• una rebanada
• una rebanada de pan

A person in Spain would understand this perfectly, and it is grammatically correct.

That said, in real situations, native speakers might choose slightly different phrases depending on the context:

• Solo quiero una rebanada de pan con queso.
  Correct and clear.

• Solo quería una rebanada de pan con queso.
  Softer, often more natural in a request.

• Me gustaría una rebanada de pan con queso.
  Polite.

• Solo quiero un poco de pan con queso.
  More natural if the exact idea of one slice is not important.

So the sentence is fine, but everyday speech may use a softer verb or a slightly different noun phrase depending on the situation.