In a large pot or Dutch oven, heat the olive oil and butter over medium heat. Add the chopped onion and cook until softened, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant.

Add the peeled and chopped potatoes and carrots to the pot. Stir to coat them with the aromatics. Pour in the vegetable broth and add the dried thyme, salt, and pepper.

Bring the soup to a boil, then reduce the heat to low, cover, and let it simmer for 20-25 minutes, or until the potatoes and carrots are very tender when pierced with a fork.

Remove the pot from the heat. Use an immersion blender to blend the soup directly in the pot until it's completely smooth. Alternatively, you can carefully transfer the soup in batches to a regular blender and blend until smooth. Return the blended soup to the pot.

Stir in the heavy cream and gently heat the soup over low heat. Do not let it boil after adding the cream. Taste and adjust seasoning with more salt and pepper if needed. Serve hot, garnished with fresh herbs if desired.