Start by bringing a large pot of salted water to a boil. Add the cauliflower florets and cook for about 5-7 minutes, or until they are tender but still firm. Drain and set aside.
In a skillet, heat the olive oil and butter over medium heat. Add the diced onion and minced garlic, sautéing until the onion is translucent and fragrant, about 3-4 minutes.
In a mixing bowl, combine the sautéed onions and garlic with the milk, sharp cheddar, mozzarella, parmesan, cream cheese, black pepper, and salt. Whisk until smooth and creamy. If the sauce is too thick, add a little more milk until you reach your desired consistency.
In a large mixing bowl, gently toss the cooked cauliflower with the cheese sauce until well coated. Transfer the mixture to a greased baking dish. Top with additional cheese if desired, and bake in a preheated oven at 350°F (175°C) for 15-20 minutes, or until the top is golden and bubbly.
Remove from the oven and let it cool for a few minutes before serving. Enjoy your Low-Carb Cauliflower Mac and Cheese hot, garnished with fresh herbs if you like!