Preheat the oven to 400°F or 200°C, slice the top from the garlic bulb, drizzle it with half the olive oil, and wrap it in foil.
Roast the garlic for 30 to 35 minutes until the cloves become soft, golden, and easy to squeeze from their skins.
Meanwhile, heat the remaining oil in a large pot, add the onion, and cook for about 4 minutes.
Add the cauliflower, potato, vegetable broth, thyme, salt, and black pepper, then bring everything to a boil.
Reduce the heat, cover, and simmer for 18 to 20 minutes until the cauliflower and potato are very tender.
Squeeze the roasted garlic cloves into the pot, then blend the mixture until smooth and creamy.
Stir in the milk and lemon juice, warm gently for 2 to 3 minutes, and serve without boiling.