In a large pot, melt the butter over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes.
Add the peeled garlic cloves to the pot and cook for another 2-3 minutes, stirring frequently to avoid burning.
Pour in the broth and bring the mixture to a boil. Reduce the heat and let it simmer for 20 minutes, allowing the flavors to meld.
Remove the pot from heat and use an immersion blender to puree the soup until smooth. Alternatively, carefully transfer the soup in batches to a blender and blend until smooth.
Return the pureed soup to the pot and stir in the heavy cream, salt, pepper, thyme, and lemon juice. Heat gently over low heat until warmed through, about 5 minutes.
Serve hot, garnished with fresh parsley.