Preheat oven to 425 degrees F. Spray a large (17-inch) baking sheet with nonstick spray.
Place the halved brussels sprouts onto the baking sheet, cut-side down. Discard any loose leaves. Top with sliced onions.
Melt the butter in a microwave-safe glass measuring cup. Add olive oil, parsley, minced garlic, salt and pepper. Stir well.
Drizzle the butter mixture over the brussels sprouts and give the pan a gently stir, making sure to keep the brussels cut-side down.
Spray sprouts with nonstick cooking spray - this ensures every bit is coated with oil.
Bake 20 minutes. During the last 5 minutes of roasting, grate the pepper jack cheese.
Please keep in mind that bake time may vary based on the size of the brussels sprouts. If they are on the smaller side, check at 12-15 minutes.
Remove the sprouts from the oven. They will have shrunk while roasting, so using a spatula, shift the brussels to one end, creating a single, yet crowded, layer. This will ensure all of the brussels are covered with the cheese and not the pan.
Top with cheese and return to the oven under the broiler on LOW for 2-4 minutes, or until the cheese has melted and starting to brown.
Remove and garnish with freshly chopped parsley, red pepper flakes, and black pepper. Season with additional salt, if preferred. Serve hot.