Melt the butter in a large soup pot.
As the butter bubbles, add the onions. Sauté until the onions begin to brown, taking care that they do not burn.
Add the mushrooms, stir, and cook for about 5 minutes.
Add the dill, paprika, salt, and broth. Stir well.
Turn the heat to low and simmer for 15-20 minutes.
In a small bowl, combine the cream and the thickener. Stir well to ensure no lumps.
Slowly add the cream mixture to the soup, stirring all the while. Continue to stir for another couple of minutes while the soup thickens. Do not boil. Add lemon juice.
Make any necessary adjustments on the thickness. Add more broth to thin it down; add cream and thickener to thicken it.
When the soup is your desired consistency, adjust for salt and pepper.
Ladle the soup into warm bowls and garnish.
Serve with freshly ground pepper.