Preheat oven to 375ºF.
Bring a couple of inches of water to a boil, add cauliflower, salt and cook until tender. Drain, puree and reserve.
Place a large saucepan over medium-high heat with 2 turns of the pan of EVOO, about 2 tablespoons. Add 2 tablespoons of butter and melt, then add the onion and garlic, season with salt and pepper, and cook until tender, 4-5 minutes.
Sprinkle the flour over the onions and cook for about 1 minute. Whisk in the stock and milk, bring up to a bubble, then reduce the heat to medium and simmer until thickened, about 5 minutes.