While many types of Korean kimchi require slow fermentation, this cucumber and scallion version can be eaten the same day it’s made. A splash of fish sauce adds depth and umami and is balanced by an equal amount of rice vinegar, which keeps the flavor fresh and bright and adds a hint of sweetness. Korean red pepper flakes, called gochugaru, are seedless, deep-red in color and fragrant. Look for them in Asian markets or in well-stocked supermarkets; if not available, red pepper flakes are a decent stand-in, but since they’re more aggressive in spiciness, use a little less. The kimchi will keep in an airtight container in the refrigerator for up to two days; serve chilled or at room temperature.