Grate the cheese and set aside. Chop the green onion and set aside.
Start a large pot of water to boil on the stove.
Peel garlic cloves and chop in half. Chop cauliflower into small chunks. Peel and chop potatoes into medium-sized chunks.
Add garlic and potatoes to boiling water. After 10 minutes, add the cauliflower as well and cook for another 10 minutes.
Meanwhile, heat the milk, either in a pot or in the microwave.
Drain the potatoes and cauliflower. Mash them. Add hot milk, oil, and salt and pepper. Then stir in cheese and green onion. Taste and adjust seasoning as desired.