For the soup:
1 pound russet potatoes, peeled and cubed
1 head of cauliflower, chopped into florets
4 tablespoons olive oil, divided
2 tablespoons unsalted butter
1 leek, sliced into half-moons and rinsed
1 medium onion, chopped (about 1 cup)
2 cloves garlic, minced
4 cups good-quality chicken stock
1/2 cup whole milk
1/2 cup crème fraîche
4 ounces grated sharp cheddar cheese (about 1 packed cup), optional
Kosher salt and freshly ground black pepper
For the dill whipped cream:
1/2 cup heavy cream
1 heaping tablespoon chopped dill
Kosher salt, to taste
For the soup, preheat oven to 450°F.
Toss the potatoes with 2 tablespoons of olive oil and season with salt and pepper. Roast for 15 minutes. Meanwhile, toss the cauliflower with remaining 2 tablespoons of olive oil and season with salt and pepper. Add the cauliflower to the pan with the potatoes, stirring to combine. Cook for an additional 25 minutes, stirring midway through cooking, until vegetables are soft and golden-brown.
In a Dutch oven or soup pot, melt butter over medium heat. Add the onions and leeks and sauté until tender and translucent, about 6 minutes. Add garlic and cook for another minute. Add the roasted vegetables and stock to the Dutch oven and season with a little salt and pepper. Bring the liquid to a boil, reduce the heat to low, and simmer for 10 minutes.
To purée the soup, either run it through a food mill fitted with the medium disc or process in batches in a blender. If you want a very smooth soup, push the purée through a fine mesh sieve or chinois, if desired.
Return the soup to the Dutch oven. Add milk and crème fraîche and rewarm over medium-low to medium heat. Whisk in cheese until completely melted, if using. Season the soup with kosher salt and pepper, to taste. (If making the soup in advance, leave out the milk, crème fraîche, and cheese until ready to reheat and serve.)
For the dill whipped cream, add heavy cream to a clean mixing bowl. Using a balloon whisk, whip the cream vigorously by hand until soft peaks begin to form. (I like a thickened, yet still malleable texture.) Gently stir in the dill and a generous pinch of kosher salt. Chill for about an hour to allow flavors to marry. If the whipped cream gets too thick, just fold in a couple splashes of cream until it reaches the desired texture.
Serve the soup with a dollop of dill whipped cream, coarsely ground black pepper, and a drizzle of good olive or walnut oil.