Long story short, I decided to substitute the regular onion for red one and the quark for smooth goat cheese. The red onions are slightly sweeter than regular ones and goat cheese works well with sweetness and balances it awesomely (I didn't want to make a dessert, after all). I think I did just fine, thank you. So the filling is:
- 200 g sweet potatoes, cooked, pureed
- 200 g white goat cheese (if given a chance, use one that's a bit harder, more like feta)
- 100 g red onions, chopped finely
They are pretty flavourful, they can work as a single dish, but they're also awesome as a side, which I tried out with steak and basil sauce. Enjoy!