In a small dry skillet, toast the chiles and Sichuan peppercorns over medium heat for 2 minutes, or until fragrant (but not browned). Transfer to a spice grinder and coarsely grind them, about 5 seconds. Put the ground spices in a small stainless steel bowl.
Heat the sesame oils in the skillet over medium- high heat. Add the garlic, if using, and cook for 1 minute. Pour half of the hot oil over the spice mixture and stir for about 30 seconds to combine well. Add the remaining half and mix well. Let cool to room temperature. Pour the oil through a fine-mesh strainer.
Discard the garlic (if used) and ground spices. Transfer the strained oil into a glass or metal container with a tight-fitting lid. It will keep for up to 3 months in the refrigerator.