4 cups roughly diced potatoes (red-skinned new potatoes or Yukon Gold)
4 cups cauliflower florets
1/4 tsp each: kosher salt and fresh black pepper
1 medium onion, peeled and roughly chopped
1/2 tsp dried thyme (or 1 tsp fresh thyme leaves)
1/4 tsp cayenne pepper
1/4 tsp dry mustard (I use Colman's)
1 qt chicken stock (homemade or low-sodium store-bought)
1/4 to 1/2 cup shredded or grated cheese (I use extra-sharp white Cheddar), plus additional for garnish
Preheat oven to 400°F.
Line a rimmed baking sheet with aluminum foil, and spread the potatoes and cauliflower in a single layer. Drizzle with 2 teaspoons of olive oil, and sprinkle on salt and pepper. Use your hands to gently toss everything about, then smooth into a single layer again.
Roast the vegetables at 400F for 20 minutes.
In a Dutch oven or heavy soup pot, heat 1 Tbsp olive oil over low heat. Sauté the onion for 2 minutes, until translucent.
Add the roasted vegetables to the soup pot, along with the thyme, cayenne and dry mustard. Stir, and cook for 1 minute. Pour in the chicken stock, raise heat to high, and bring to a boil. Then, reduce heat to simmer, cover the pot, and cook on lowest heat for 25 minutes, until the vegetables are very tender.
Remove the pot from the heat, and purée the soup with an immersion blender. (Or, let the soup cool slightly, process in batches in a standing blender or food processor, and return the soup to the pot.)
Return the pot to the stovetop. Stir in the shredded cheese, until it melts. Taste, and adjust seasoning if necessary with salt and pepper (you will likely not need more salt, as there's salt in the cheese).
Serve hot, garnished with additional cheese, if desired. The soup can be cooled completely, and refrigerated for up to 3 days.