Lanzhou Chile Oil

Fill a wok or small saucepan with the neutral oil and secure a deep-fry thermometer on the side. Add the yellow onion, green onion, ginger, cinnamon, cardamom, star anise, fennel seeds, and peppercorns. Place the pan over medium heat and warm the oil until it reaches 300°F (no higher, or the oil will taste burnt) and the onions begin to caramelize at the edges, 12 to 14 minutes. Remove from the heat.

Line a fine-mesh strainer with a double layer of cheesecloth and set over a heatproof bowl. Pour the oil mixture through the strainer, then gather up the sides of the cheesecloth and tie closed with kitchen twine to form a sachet. Add the sachet, chile flakes, white and black sesame seeds, and soy sauce to the oil and stir to combine. Let cool to lukewarm, then cover and let sit overnight at room temperature. Remove the sachet and gently squeeze any oil in it back into the bowl. Discard the sachet.

Transfer the chile oil to an airtight container and store in the refrigerator for up to 3 months.