Marinade:
Steep the tea in boiling water for 5 minutes.
Add the other marinade ingredients, except for the lime juice, and mix well.
Place the mackerel fillets in a dish and pour the marinade over them.
Cover with plastic wrap and refrigerate for at least 2 hours.

Radish salad:
In a bowl, mix the radishes, olive oil, and chives. Season with salt and pepper and refrigerate.

Cooking:
Preheat the oven to 450 F (230 C).
Drain the mackerel and place them on a baking sheet lined with a silicone mat.
Put the marinade in a saucepan and bring to a boil.
Reduce by half, then strain through a sieve.
Add the lime juice and mix.

Brush the mackerel fillets with the marinade, then bake for 6 minutes.
Halfway through cooking, brush each fillet with a little marinade.

Serve the fillets straight from the oven with the radish salad, topped with the reduction.