Roast Barramundi with Potatoes and Celeriac

Ingredients

• 400 gm kipfler potatoes, scrubbed, cut into 1cm-thick slices
• 125 ml olive oil (½ cup)
• ½ small Spanish onion
• 1 small celeriac (about 300gm), cut into julienne
• 1 tbsp red wine vinegar, or to taste
• ½ cup coarsely chopped flat-leaf parsley
• 4 skinless barramundi fillets (about 180gm each)
• 100 gm chilled butter, coarsely chopped
• 40 gm salted capers, rinsed
• 1 garlic clove, thinly sliced
• To serve: lemon wedges