Preheat oven to 400 degrees F. Lightly coat shallow baking pan with nonstick cooking spray. Place cauliflower in pan; drizzle with oil. Toss to coat. Roast 30 to 40 minutes or until just browned, stirring occasionally.
Place potatoes in large saucepan with salted water to cover. Bring to boiling; reduce heat. Simmer, covered, 15 to 20 minutes or until very tender. Drain; return potatoes to saucepan. Mash with potato masher; set aside.
Place cauliflower in food processor; cover and process until very finely chopped, adding 2 Tbsp. of the milk. Add cauliflower mixture to potatoes in saucepan. Add remaining milk, the coconut oil, salt, and black pepper. Heat through over medium heat, stirring frequently. Top with green onions.