Learn Portuguese (Portugal): As maçãs assadas são a sobremesa.

Assadas is an adjective describing the apples (as maçãs). Because maçã is a feminine word and we are talking about more than one (plural), the adjective must also be feminine and plural, ending in -as.

Why do we use são (from the verb ser) instead of estão (from the verb estar)?

We use são because we are defining what the apples are. The verb ser is used for identity and definitions (they are the dessert). The verb estar is used for temporary states or locations, so using it here would sound strange, as if the apples are only temporarily acting as the dessert.

Does assadas mean 'roasted' or 'baked'?

It can mean both! In Portuguese, the verb assar is used for both roasting (like meat) and baking (like apples, potatoes, or fish in an oven). You tell which english word fits best based on the context.

The word sobremesa looks like it is made of two other words. Is it?

Yes! It comes from sobre (on or over) and mesa (table). It literally means what is placed on the table after the main meal is cleared away. This makes the word for 'dessert' very easy to remember!