Салями оказалась солёной, поэтому к брынзе я подала ещё помидоры и огурцы.

Салями is a borrowed, indeclinable noun in Russian, and it is usually treated as feminine.

That is why the sentence uses feminine forms:

• оказалась — feminine past tense
• солёной — feminine singular

So Russian speakers understand салями as something like a type of колбаса (which is feminine), and the agreement follows that pattern.

Оказалась means turned out to be or proved to be.

So:

• Салями была солёной = The salami was salty.
• Салями оказалась солёной = The salami turned out to be salty.

Using оказалась adds the idea that this was discovered or became apparent, not just stated as a fact.

After verbs like быть, стать, оказаться, Russian often uses the instrumental case for the predicate noun or adjective.

So:

• оказалась солёной = turned out to be salty

Here солёной is the instrumental feminine singular form of солёная.

This is very common with оказаться.