Preheat the oven to 400° F. Have a 9-inch by 13-inch baking dish ready.
Wash the sprouts and shake off the excess water.
Slice the Brussels sprouts: if they're very large slice them into quarters. Smaller sprouts can be cut in half.
Place the cut sprouts in the baking dish. Drizzle the oil over the top and toss the sprouts to coat them.
Sprinkle in the chopped rosemary, minced garlic, brown sugar, salt, and pepper over the top and toss the sprouts gently in the seasonings.
Place the pan in the preheated oven and roast them for 25-30 minutes, stirring them 2 or 3 times, until the sprouts are tender with some browning on the edges.
Take the sprouts out and serve them while they're hot.
Notes
After slicing the sprouts, if they have craggy stems or large cores you can slice these out.
If the sprouts are especially large, slice them into quarters, Smaller sprouts can just be sliced in half.