1. Preheat oven to 350℉.
2. Use a serrated knife to remove the top of the garlic. Place the garlic onto a small piece of aluminum foil and drizzle with olive oil. Lightly wrap the garlic in the foil.
3. Roast the garlic in the preheated oven for 45-60 minutes, or until the garlic cloves are soft. Gently squeeze the garlic cloves into a small bowl and use a fork to mash the garlic. Add the smashed garlic to a large mixing bowl, then set aside. Leave oven on and increase temperature to 400℉.
4. Line a baking tray with additional aluminum foil and lightly grease with cooking spray.
5. Place the cauliflower, cut-side down, onto the tray. Lightly grease with olive oil or cooking spray. Season with salt and pepper, to taste.
6. Roast cauliflower in the preheated oven for 25-30 minutes, or until the cauliflower is lightly browned and soft. Use a knife to remove the cauliflower florets and add them to the large bowl with the garlic cloves.
7. Meanwhile, skin the potatoes and bring a large pot of water to boil. Cut the potatoes into large chunks and add them to the boiling water. Cook for about 10-15 minutes, or until soft. Drain the water from the potatoes.
8. Place the cooked potatoes into the large mixing bowl with the garlic and cauliflower. Add the milk, butter, 1½ teaspoons salt, and 1 teaspoon black pepper. Use a potato masher to roughly mash everything together then use an immersion or hand blender to blend the potato mixture into a puree.

Serve immediately while warm. Enjoy!