Preheat oven to 325°. Rub fish with 2 Tbsp. oil on a parchment-lined rimmed baking sheet. Using a Microplane, finely grate grapefruit directly over fish 2–3 times so each fillet is seasoned with a bit of zest; season with salt. Bake fish until the flesh is opaque and begins to flake, 12–16 minutes (thicker pieces may take longer).

Meanwhile, separate cabbage leaves and tear into 3"–4" pieces. Heat 1 Tbsp. oil in a medium skillet over medium-high. Add cabbage and cook, undisturbed, until about half of the leaves are charred in spots, about 3 minutes. Season with salt and give the skillet a toss a few times. Continue to cook, undisturbed, until all leaves have a bit of char on them, about 2 minutes more. Divide cabbage among plates.

Using a sharp knife, cut all peel and white pith from grapefruit; discard. Thinly slice grapefruit into ½"-thick rounds.

Add grapefruit rounds to plates with cabbage; season with salt. Break fish into large pieces and divide among plates. Top with mint. Drizzle with more oil, then season generously with pepper.