Heat the oven to 425°F.
Spread the cauliflower on a large baking sheet.
Toss it with 2 tablespoons olive oil, salt, pepper, thyme, and smoked paprika.
Cut the top off the garlic head.
Drizzle the garlic with 1 teaspoon oil.
Wrap the garlic in foil and place it on the pan.
Roast the cauliflower for 30 to 35 minutes.
Roast the garlic for 35 to 40 minutes.
Stir the cauliflower once halfway through for even browning.
Melt the butter in a large pot over medium heat.
Add the diced onion and cook 6 to 8 minutes.
Stir often until the onion turns soft and lightly golden.
Pour in the broth and bring it to a gentle simmer.
Add the roasted cauliflower to the pot.
Squeeze in the roasted garlic cloves.
Simmer everything for 10 minutes.
Blend until smooth with an immersion blender.
Stir in the cream, Parmesan, and lemon juice.
Taste, then add more salt or pepper if needed.
Warm it gently for 2 more minutes.
Serve hot with toppings you like.