Lekker is an adjective here. It describes the noun phrase by saying what the side dish is like: tasty, nice, good.
Afrikaans learners should know that lekker is a very common word and can be used in a wider range of ways than English tasty. Depending on context, it can mean:
In this sentence, it most naturally means tasty or delicious.
No. That is one of the most useful things to know about Afrikaans.
Lekker can be used for food, but also for many other things:
So it is a very flexible everyday word.
Yes. The plural is usually bykosse.
Notice something important: in Afrikaans, is is still used with plural subjects in the present tense in sentences like this. Afrikaans verbs usually do not change much for person or number the way English verbs do.