Learn Finnish: Tämä pääruoka on yksi herkullisimmista annoksista. - This main course is one of the most delicious dishes.

This is the Finnish way to say 'one of the...'. After yksi (one), you use the elative plural case (ending in -ista or -istä) for both the superlative adjective and the noun. It literally translates to 'one out of the most delicious dishes.'

How is the superlative word herkullisimmista formed from the basic word herkullinen (delicious)?

To make the basic superlative 'most delicious', the ending becomes -isin, making herkullisin. When you put this into the plural elative case, the -in changes to the stem -immi-, and then you add the plural elative ending -sta. So the chain is: herkullinen -> herkullisin -> herkullisimmista.

Doesn't annos usually mean 'portion'? Why is it translated as 'dishes' here?

Yes, annos primarily means 'portion' or 'serving.' However, in a restaurant context, it is very commonly used to refer to a 'dish' on a menu (a specific plated meal). You could also use ruoista (from ruoka, food), but annoksista is exactly what you would say when talking about the specific meals served at a restaurant.

Is pääruoka always written as one word?