Combine all of the ingredients in a medium saucepan and bring to a simmer over low heat until the chilis are tender but not browned, about 30-40 minutes. Let cool.
Using a slotted spoon, transfer the chilis into and air tight container. Pour the cooking oil on top, seal and refrigerate for up to two weeks.
*note Fresno and shishito peppers are mild in spice, if you want to use a spicier chili use jalapeno or serrano.