Rowan Berries are very healthy. The astringent taste fades with freezing, so the best time to pick them is winter.

But.

If in your home forest are living greedy birds, you should pick them as soon as possible and leave in a deep freezer at least for 3 hours.

more options:
soak berries in salted water (1 teaspoon salt in 1-litre water)
or keep berries  3- 4 minutes in boiling water

Mash some berries and mix with sour cream
season with salt, sugar and pepper
Chop (red) onion or leek
add marinated or salted mushrooms
and cottage cheese
decorate with rowan berries