At least 35 to 55 minutes before serving
Preheat a water bath to 114°F (45.6°C).
Clean the monkfish and remove all membranes and connective tissue. Cut into 1" (25mm) medallions. Lightly salt and pepper the medallions then zest the lemon over the top. Place the monkfish in a sous vide bag with the butter then seal. Cook for 15 to 35 minutes, until heated through.
At least 4 hours before serving
Remove some of the powdery coating from the kombu by gently wiping it with a damp cloth or paper towel. Add the kombu to a pot and cover with the water. Soak for 45 minutes.
Slowly heat the water to 160°F (71.1°C) over medium heat, this should take about 10 minutes. Remove the kombu from the pot and discard.
Turn the heat up to high and bring to a boil. Add the bonito flakes and remove from the heat. Let steep for 5 minutes. Strain the liquid to remove the bonito and any floating particles. Let the dashi cool and then place in the refrigerator to fully chill.
The dashi can be stored in the refrigerator for a week.
Remove the fish from the bag and pat dry. Briefly sear the medallions on one side.
Place the fish in the middle of a shallow bowl with the seared side up. Set some snow peas, radish slices and cucumber around the fish. Sprinkle some mint leaves on top of the fish. Pour the dashi around the fish. Drizzle some mirin and sesame oil in the dashi, squeeze a lemon wedge over the fish then serve.