1. melt butter over medium heat, and add onions.
2. Cook until slightly translucent.
3. Add mushrooms, and cook until done.
4. Mix in the soup powder or the flour and cook for about a minute.
5. Gradually, add the whipping cream while stirring constantly.
6. Cook until the mixture has a sour cream like consistency, while simmering, add the optional cheese and the dill.
7. Season with salt and pepper.