In a large mixing bowl, combine cauliflower florets and garlic cloves. Drizzle with olive oil, and season with salt and pepper. Toss until evenly coated.
Spread the cauliflower and garlic in a single layer on a baking sheet. Roast in the oven for about 25-30 minutes, until golden brown and tender, flipping halfway through.
In a large pot over medium heat, add chopped onion and sauté until translucent, about 5-7 minutes.
Once roasted, add the cauliflower and garlic to the pot with the onions. Pour in the vegetable broth and bring to a simmer.
Stir in the dried thyme and lemon juice, then let it simmer for another 10 minutes to meld the flavors.
Using an immersion blender (or in batches using a regular blender), puree the soup until smooth.
Stir in the heavy cream and grated Parmesan cheese, cooking until the cheese has melted and the soup is heated through. Adjust seasoning with salt and pepper if needed.
Serve warm, garnished with fresh parsley and an extra sprinkle of Parmesan cheese on top.
Notes
For a vegan version, substitute heavy cream and Parmesan with plant-based alternatives.