Trim the stem ends of the Brussels sprouts, remove loose outer leaves, and halve each one through the stem. Pat dry.
In a bowl, toss the halved sprouts with the oil, salt, and pepper until evenly coated.
Preheat the Blackstone griddle to medium-high and let it come to an even temperature.
Place the sprouts cut-side down in a single layer, leaving space between them. Don't stir.
Sear undisturbed for 4–5 minutes, until the cut sides are deeply browned and caramelized.
Flip the sprouts, add a splash of water (about 2 tablespoons) to the griddle, and cover with a basting dome for 3–4 minutes to steam the centers until tender.
Remove the dome, toss with any optional garlic, Parmesan, or balsamic, and serve hot.