Roasted Cauliflower Soup with Cumin

medium head of cauliflower (1 1/2 pounds)—halved, cored and cut into 1 1/2-inch florets

cumin seeds

curry powder

sunflower or grapeseed oil

Kosher salt

Freshly ground pepper

small onion, diced (1 cup)

unsalted butter

bay leaf

water

whole milk