Roughly chop the potatoes and cauliflower. Place potatoes in a medium pot first, then add cauliflower on top. Add water and steam/boil for about 20 minutes, until everything is soft to the touch.
Once potatoes and cauliflower are soft, remove from pot and place in a 9x13 dish. Use a potato masher to smash them up and mix together. Add 1/2 the cheese, 1/2 the bacon, 3 cloves of minced garlic, and lots of salt and pepper. Mix all ingredients well. (You can use a large mixing bowl if you prefer, but I was trying to minimize the dirty dishes!)
Smooth ingredients out and sprinkle remaining cheese, bacon, and the green onions.
Either cover the casserole with plastic wrap and store in the fridge for later, or pop the pan into the oven at 350° for about 20 mins.