Zest the grapefruit and lemon into a large bowl.
Cut off the top, bottom, outside peel and pith from the grapefruit. Over a small bowl, use a knife to separate the segments. Cut as close to the membranes as possible. Lift out the segments and place on a plate. When finished, squeeze all the juice from what is left of the grapefruit.
Using a mandoline or sharp chef’s knife, thinly slice the fennel. (alternately slice/shred with a food processor). Place the fennel slices in the large bowl with the zest.
Add the grapefruit juice, dill, sea salt and pepper flakes to the bowl; stir to combine. Add the olive oil and combine some more. Finally, add the grapefruit segments and gently stir until coated.
Season the cod with sea salt and pepper. Place a large skillet over medium-high. Add the oil and heat until almost smoking. Place cod in the pan and cook for 2 to 4 minutes per side, depending on thickness. The cod should be lightly browned and flake easily.
Divide the slaw onto 2 plates. Top with the cod. Garnish with dill fronds. Serve immediately.