Bring a large pot of water to a boil; add Brussels sprouts and return to a boil. Cook, for about 20 minutes, or until sprouts are tender but firm (If using frozen, cook according to the package directions). Drain.
Heat butter in a skillet over medium-high heat; add sprouts and garlic, and saute 5 minutes, stirring, until garlic is soft and fragrant.
Add cider to skillet and turn heat to high, stirring constantly, until cider is reduced in volume by half, making a sauce.
Add salt and pepper to taste and serve immediately.