400 gmkipfler potatoes, scrubbed, cut into 1cm-thick slices125 ml (½ cup)olive oil½ small Spanish onion1small celeriac (about 300gm), cut into julienne1 tbspred wine vinegar, or to taste½ cupcoarsely chopped flat-leaf parsley4skinless barramundi fillets (about 180gm each)100 gmchilled butter, coarsely chopped40 gmsalted capers, rinsed1garlic clove, thinly slicedTo serve:lemon wedges
Cook potato in a saucepan of boiling salted water until tender (15-20 minutes). Meanwhile, heat 90ml oil in a large frying pan over medium-high heat, add onion and celeriac and toss occasionally until tender and lightly golden (4-5 minutes). Drain potato, add to pan and toss occasionally until golden (4-5 minutes). Add vinegar, season to taste, toss in half the parsley and set aside.
Preheat oven to 200C. Heat remaining oil in a large ovenproof frying pan over medium-high heat, add barramundi and cook, turning once, until browned (2-3 minutes each side), transfer to oven and roast until just cooked through (2 minutes).
Cook butter in a small saucepan over medium-high heat until nut-brown (3-4 minutes), then remove from heat, add capers, garlic and remaining parsley, season to taste and stir to combine. Spoon the butter over servings of barramundi and potato and celeriac and serve with lemon wedges.