Cook/steam Brussels sprouts in salted water until tender about 5-7 minutes (if using fresh, will take a bit longer). Drain and place hot drained Brussels sprouts in a serving dish.
Meanwhile, in a saucepan, over medium heat, sautee onions and garlic in the butter until onions are translucent; about 4-5 minutes. Add cream, parmesan, salt and pepper and stir. Cook over medium heat, stirring frequently, until cheese is fully melted and sauce begins to thicken.
Pour sauce over Brussels sprouts and serve immediately.