Heat oil in a large pot over medium. Add onion, fennel, celery, and salt; cook, stirring often, until vegetables are translucent, about 4 minutes. Add Pernod and red pepper flakes; cook, stirring, until Pernod is evaporated, about 1 minute. Pour in 5 cups water and bring to a simmer. Reduce heat to medium-low and cook 45 minutes. Strain through a fine-mesh sieve into a medium heatproof bowl; discard solids.Do Ahead: Potlikker can be made 5 days ahead. Let cool. Transfer to an airtight container and chill, or freeze up to 3 months.