Preheat the oven to 400ºF. Prepare a baking sheet: line a baking sheet with parchment paper OR foil sprayed with nonstick cooking spray. You can also spray the baking sheet directly. Set aside.
Prepare the Brussels sprouts and garlic: Scrub the Brussels sprouts under warm water to remove any dirt. Pat dry with a towel. Use a sharp knife to cut both ends off of each Brussels sprout. If some of the leaves fall off, don't worry! You'll roast these, too. Then, cut them in half longways (hot-dog style). The goal is to get all of the Brussels sprouts around the same size so that they roast evenly. If you have some extra-large sprouts, you may need to quarter them. Use a sharp knife to cut both ends off of a head of garlic. Transfer the Brussels sprouts and head of garlic to the prepared baking sheet.
Add the minced onion to the baking sheet.
Drizzle the veggies with a generous amount of olive oil salt and season with salt, pepper, and garlic powder. Use your hands to massage the oil and spices into the veggies.
Evenly spread the Brussels sprouts out onto the baking sheet. Make sure they don't touch - if they overlap at all, they will not roast evenly. Make sure the head of garlic is open-face down.
Bake for 25-30 minutes, tossing the Brussels sprouts and flipping the head of garlic every 10 minutes. Brussels sprouts are done when they're brown and crispy.
Let cool for 10 minutes. Use your hands to squeeze the garlic cloves out of the casing. Toss the cloves with the Brussels sprouts and enjoy.