Preheat the oven to 450 degrees F. Spray a large baking sheet with nonstick spray.
Spread the cauliflower onto the baking sheet in a single layer. Drizzle with 2 tablespoons of the oil and some salt and pepper. Toss to coat well. Roast, stirring once or twice, until the cauliflower is just tender, about 20 minutes.
Meanwhile, heat the remaining tablespoon oil with the butter in a large nonstick skillet over medium-high heat. Add the garlic and onions and cook, stirring occasionally, until softened, about 8 minutes. Stir in the cream cheese and heavy cream and bring to a simmer. Add the cauliflower, tossing gently to coat well. Transfer to a serving bowl and sprinkle with the parsley.