Equally stagger two racks in the oven and preheat it to 425F.
Toss the sprouts with 2 tablespoons olive oil, season with a pinch of salt, and scatter on two sheet pans. Roast until the sprouts are tender and beginning to brown on the edges, 20 to 22 minutes.
In a small bowl, whisk together the vinegar, honey, and pepper flakes and season with a pinch of salt and a few cracks of black pepper.
Set a large pan over medium heat and add 2 tablespoons olive oil. Add the red onion, season with a pinch of salt, and cook, stirring occasionally, until softened, about 8 minutes.
Add the cornmeal and toast until it's beginning to turn a deep golden brown, 7 to 8 minutes. Drizzle in 2 tablespoons of the vinegar mixture, turn off the heat, and shake the cornmeal into a crumble.
When the sprouts are out of the oven, toss with the remaining vinegar mixture on the sheet pan. Transfer to a serving dish and scatter the cornmeal mixture over everything.