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