Heat olive oil and butter in a large pot over medium-high until foamy.
Add mushrooms and 1/2 teaspoon salt. Cook, stirring, until they release liquid and brown, 8–10 minutes.
Stir in onion. Cook until softened and lightly golden, 4–5 minutes.
Add garlic and thyme. Cook until fragrant, about 30 seconds.
Sprinkle flour over vegetables. Stir constantly for 1 minute to cook the flour.
Pour in white wine. Scrape the pot to release browned bits; simmer 1 minute.
Gradually stir in broth. Bring to a gentle boil, then reduce to a simmer.
Simmer uncovered until slightly thickened and flavors meld, 12–15 minutes.
Blend part of the soup with an immersion blender to your preferred texture.
Stir in cream and lemon juice. Season with remaining salt and pepper to taste; warm gently.
Ladle into bowls and finish with parsley and extra pepper.