Preheat oven to 350°. Warm 2 tablespoons olive oil in a 4- to 6-quart pot over medium heat. Add onion and sea salt; sauté 5 minutes. Stir in garlic and sauté a minute more. Add mushrooms and stir until coated with oil. Add cauliflower and water, cover, bring to a boil, reduce heat, and simmer 15 minutes.