On peut aussi garnir le dessus avec quelques amandes pour que le dessert soit plus joli.

Yes, on often corresponds to English one in a general sense, but in everyday French it is very commonly used where English would say you, we, or people.

In On peut aussi garnir..., on means something like:

• You can also...
• We can also...
• It’s also possible to...

It is much more natural here than using a more specific subject like nous.

Garnir means to garnish, to decorate, to top, or to add something onto a dish.

In this sentence, it suggests adding something on top for appearance or finishing the dessert. So garnir le dessus avec quelques amandes means to top the surface with a few almonds or to decorate the top with a few almonds.

Dessus can be used in different ways in French.

• le dessus = the top / the upper part / the surface
• dessus by itself often appears in expressions like mettre dessus = put on top of it

Here, garnir le dessus means decorate the top or decorate the surface.
The article le is needed because dessus is being used as a noun.

Quelques amandes means a few almonds.

French uses:

• des amandes = some almonds
• quelques amandes = a few almonds

Using quelques is a bit more specific: it suggests a small number rather than just an unspecified amount.

Use pour when it is followed by an infinitive, usually when the subject stays the same:

• Je viens pour manger. = I’m coming to eat.

Use pour que when it introduces a whole clause with its own subject and verb:

• ...pour que le dessert soit plus joli. = ...so that the dessert is prettier.

Here, le dessert is the subject of the second verb, so French uses pour que.

Because pour que triggers the subjunctive in French.

The verb être in the subjunctive is:

• que je sois
• que tu sois
• qu’il / elle / on soit
• que nous soyons
• que vous soyez
• qu’ils / elles soient

So pour que le dessert soit plus joli is correct because after pour que, French normally uses the subjunctive.

At first, it is perfectly fine to learn pour que + subjunctive as a pattern.

So you can memorize:

• pour que + verb in the subjunctive

Examples:

• pour que ce soit plus facile
• pour qu’il vienne
• pour qu’elle soit contente

That said, it is worth learning the subjunctive more broadly, because it appears after many common expressions, not just pour que.

Yes, literally it means more pretty, but natural English would usually say:

• prettier
• more attractive
• nicer-looking

So pour que le dessert soit plus joli means so that the dessert looks nicer / prettier.

French often uses être + adjective where English prefers look + adjective.

So:

• Le dessert est joli literally = The dessert is pretty
• but in natural English, depending on context, we may say The dessert looks pretty

That is why le dessert soit plus joli can naturally be translated as the dessert looks nicer, even though French uses be rather than look.

In On peut aussi garnir..., aussi means also.

It comes after the conjugated verb peut:

• On peut aussi... = You can also...

This is a very common position for adverbs in French: after the conjugated verb.

Compare:

• Il mange souvent ici.
• On peut aussi essayer.

It depends on context, and all three are possible in theory. In a recipe or instruction, on peut most often feels like:

• you can
• or we can

In natural English recipe language, you can also top it with a few almonds is probably the most idiomatic translation.

Yes. French offers several natural alternatives, depending on the nuance:

• ajouter quelques amandes sur le dessus = add a few almonds on top
• décorer le dessus avec quelques amandes = decorate the top with a few almonds
• parsemer le dessus de quelques amandes = sprinkle the top with a few almonds

Garnir sounds a little more culinary and polished, which fits recipe language well.