In a large pot over medium heat, melt the butter with the oil.
Add the onion and cook until soft, 6 to 8 minutes.
Add the mushrooms, 2 teaspoons salt, and cook, stirring occasionally, until the mushrooms are soft and golden, 8 to 10 minutes.
Sprinkle in the flour and cook, stirring constantly, 1 minute.
Stir in the paprika, and cook until fragrant, 30 seconds.
Slowly pour in the broth, scraping up any browned bits.
Bring to a simmer and cook until slightly thickened, 10 minutes.
Remove from heat. Whisk the cream into the soup—a little at a time—until smooth.
Stir in the lemon juice, additional salt, and pepper to taste.
Serve garnished with chives.