Preheat oven to 375 degrees. Coat a 2-quart casserole dish with nonstick spray.
Steam cauliflower until crisp-tender.
Remove and place in casserole dish. In a medium saucepan, melt the butter, add the garlic and cook until it starts to sizzle. Add the flour and stir for about 1 minute, do not brown. Slowly add the broth and milk. Cook, whisking constantly, until the mixture comes to a boil.