Trim the Brussels sprouts and cut larger ones in half, then toss them in the oil. Add them to a baking dish and roast for 15 minutes.
Meanwhile, in a bowl, whisk the miso, water, garlic and soy sauce (save the cream to use at the end), stirring to combine well, then mix in the onion.
Pour the mixture over the brussels sprouts in the baking tray and gently stir to coat. Return the tray to the oven and cook for a further 20 minutes, stirring occasionally, or until the Brussels sprouts and onions are cooked and soft. Add the cream to the baking tray and stir. Place back in the oven for a minute to heat.
Notes
Be sure to trim the Brussels sprouts before roasting to remove any tough or browned outer leaves.
Stir the Brussels sprouts halfway through roasting to ensure they cook evenly.
Don’t add the cream until the very end, as heating it for too long may cause it to separate.
Adjust the amount of miso based on your taste—start with less if you're unsure, as it can be quite salty.