Preheat the oven to 400 degrees F or 375 F convection if using multiple sheet pans.
Line a large ¾ sheet pan or two regular sheet pans with unbleached parchment paper.
Toss brussels sprouts and onions with a Tablespoon of oil and rosemary in a bowl or sheet pan. Arrange on the prepared sheet pan in one layer. Season with salt and pepper. Roast for 10-15 minutes, or until just starting to brown.
Carefully toss grapes with a few drops of oil and season with a pinch of salt. Remove sheet pan from oven. Arrange grapes on the sheet pan with the Brussels sprouts, filling any open spaces. Roast for 15 minutes, until grapes have shriveled slightly.