Preheat oven to 400° F. Set a large sheet tray out for roasting.
Rinse the cauliflower. Using a sharp knife cut around the root of the cauliflower to remove the florets. Continue working your way around the cauliflower until all florets are removed. Place cauliflower florets onto the sheet tray. Add the whole garlic cloves to the tray. Sprinkle kosher salt and drizzle 1 tbsp olive oil over top. Using clean hands mix together. Place into the oven and roast for 20-25 minutes until the cauliflower is soft tender.
Allow the cauliflower and garlic to cool slightly. Place cauliflower and garlic into the bowl of a food processor. Add 1/4 cup of milk. Process until smooth. Set aside.
Scrub the potatoes, peel and cut them into large dice.
Fill a large pot with water and heat to boiling. Add the diced potatoes. Cook the potatoes until they are fork tender, about 12-15 minutes.
Drain the potatoes and return to same pot you boiled them in.
Using a potato masher, hand held mixer or potato ricer, mash the potatoes until your desired consistency.
Add the cauliflower puree, additional 1/4 cup of milk, 1/2 tsp kosher salt and butter to the potatoes and mix to combine.
Check the seasonings and adjust the salt and pepper as needed. Serve hot and enjoy!