1/2 cup grated Parmesan cheese, plus more for garnish
Olive oil for roasting - Truffle oil (to taste, and optional)
Salt and Pepper to taste
Preheat the oven to 400 degrees.
Cut the cauliflower into even-sized pieces and spread in a single layer onto a baking sheet.
Drizzle with olive oil and season generously with salt and pepper. Roast the vegetables for about 30 minutes, turning them about halfway through until they start to brown on the edges and become soft. Set aside.
Meanwhile, in a medium-sized pot, heat a drizzle of olive oil and add the onions and a pinch of salt. Sweat the onions until they become translucent, then, add the garlic and cook for a minute more.
Add the potatoes to the pot and just enough water to cover them. Add the thyme and bring potato-onion mixture to a boil.
Reduce to simmer and cook until the potatoes are tender. Let the mixture cool a bit and working in small batches, use a blender to puree the cauliflower, potatoes and potato water.
Return to a clean pan and add the Parmesan cheese.
Heat the soup until the cheese melts. If using, add truffle oil, season to taste with salt and pepper.
Serve garnished with Parmesan cheese and a couple of fresh thyme leaves.