Preheat the oven to 425º F. Put a roasting pan in the oven to heat up while you prepare the ingredients.
Mince the garlic and dice the red onion. Add to a large bowl.
In a separate bowl, whisk together the ingredients for the sauce. Taste and adjust to your liking.
Pour the sauce into the bowl with the onions and garlic, then mix well.
Prepare the Brussels sprouts: rinse and dry them, then cut off the fibrous ends. Remove the outer layer of leaves, then cut each sprout in half. Add to the bowl with the other ingredients and stir to combine.
Carefully remove the hot roasting pan from the oven and pour the sprouts over it. They should sizzle as they hit the pan. Stir to distribute, then put them in the oven for 15 minutes.
After 15 minutes, stir them again so they cook evenly. Roast for an additional 10-15 minutes, or until brown and crispy. If they're not as crispy as you want, put the broiler on high for one minute, or until they look right.
Taste and adjust flavors again as necessary, then serve hot.