Pomegranate molasses, honey, balsamic vinegar or mulberry are wonderful here. Same with the hazelnuts--other nice options are toasted pine nuts or walnuts.
Cover a sheet pan with nonstick foil and place in the middle rack of the oven. Heat the oven, with the pan in it, to 500 degrees F.
Meanwhile, toss the Brussels sprouts in a large bowl with the olive oil, garlic powder, big pinch of salt and a few grinds of black pepper. I don't toss the onion here because the wedges are delicate and will fall apart.
When the oven is heated, remove the sheet pan and quickly spread the Brussels sprouts out on it, turning them face down. Tuck in the red onion.
Reduce the oven temperature to 450 and roast the sprouts until they are bright green, with deep golden brown and some burnished spots, about 12-15 minutes.
Drizzle the brussels sprouts and onions all over with more olive oil, pomegranate molasses and lemon juice. Toss on the sheet pan to coat. Taste and adjust seasonings as needed. Finish with the hazelnuts and serve immediately.