Arrange in a single layer on a baking sheet covered in foil, parchment, or coated in cooking spray. Bake for 12 minutes and then shake the baking sheet. Bake for 5-15 minutes longer until crispy. The cooking time varies so much because the size of the onions and your oven will make a big difference in this recipe. You want to take them out when they are beginning to brown, caramelize, and crisp up on the edges.