Prep the fish and dry it thoroughly: pat the fillets completely dry with paper towels and season both sides with 1/2 tsp of the salt, 1/4 tsp pepper and half the smoked paprika. Drying the surface removes moisture so the exterior crisps in the air fryer rather than steaming.

Make a light herb-lemon crumb: in a small bowl combine the parsley (reserve a pinch for finishing), zest of half the lemon, the remaining smoked paprika, and a drizzle (about 1 tsp) of olive oil to make a loose paste. Brush a thin layer of this herb paste onto the top (not the underside) of each fillet — the oil carries the herb aromas and the zest brightens the fish.

Prepare the potato ribbons for crisping: using a swivel peeler or mandoline, shave the potato into very thin ribbons (about 1 mm thick). Place the ribbons into a large bowl of cold water and let sit 5 minutes to leach surface starch — this step is key to achieving light, crisp ribbons instead of gummy ones. After soaking, drain and dry thoroughly in a clean towel.

Season and dress vegetables: halve the tomatoes crosswise so the cut side faces up; cut the onion into 6–8 thick wedges and separate the layers slightly. Toss the potato ribbons, tomato halves and onion wedges in a bowl with the remaining 2 tbsp olive oil, minced garlic, remaining 1/2 tsp salt and 1/4 tsp pepper. The oil promotes browning in the air fryer and the garlic will gently roast with the vegetables.

Preheat your air fryer to 200°C (390°F) for 3 minutes. Preheating gives an immediate blast of hot air so surfaces start to brown right away—important for crispness.

Air-fry the potato ribbons first: spread them in a single layer in the air fryer basket (work in batches if needed) and cook at 200°C for 6–8 minutes, shaking halfway through. Cooking the potatoes alone first allows them to crisp without moisture from tomatoes/onions softening them. Remove and keep warm on a rack so air can circulate and preserve crispness.

Air-fry the tomatoes and onions: place tomato halves cut-side down and onion wedges in a single layer and air-fry at 200°C for 6–8 minutes, turning the onion once halfway. Tomatoes should blister and blacken slightly—this concentrates their sweetness and adds smoky char notes.

Air-fry the fish: place the seasoned fillets (herb-side up) in a lightly oiled air fryer basket so they don’t touch. Cook at 200°C for 8–10 minutes, depending on thickness (about 10 minutes for a 1"/2.5 cm thick fillet). Fish cooks quickly in the air fryer; the high circulating heat forms a golden exterior while keeping the interior moist. Use a thermometer if unsure—cod is done at 62°C (145°F) internal temperature and flakes easily.

Make the lemon-butter finishing sauce: while the fish rests, melt the butter in a small skillet over low heat, add the juice of the remaining half lemon and a pinch of salt, then stir in the minced reserved parsley. Keep warm. The butter adds a silky gloss and lemon brightens the flavors—just spoon a little over the fish at plating.

Plate and finish: arrange a nest of potato ribbons on each plate, place a fillet on top, and surround with charred tomatoes and onion wedges. Spoon 1 tbsp of the lemon-butter over each fillet, sprinkle the remaining fresh parsley and an extra grind of pepper. The contrast of textures—crispy potatoes, tender fish, and juicy charred vegetables—makes the dish sing.

Serving note: serve immediately while the potatoes are still crisp. If you must hold the components briefly, keep the potatoes on a cooling rack in a warm (not hot) oven to prevent steaming and maintain texture.