Bring 2c water, 1c mirin to a boil. Add 1c brown sugar, 1/2c kosher salt and 1tbsp each of fennel seed, mustard seed, coriander seed, peppercorns and chili flakes. Return to a boil and pour over cleaned spring carrots. Chill and allow to steep for 2 days. Drain and discard liquid, but garnish carrots with seeds before service.