Before you start, preheat oven to 230°C (or 210°C fan bake). Bring a medium pot of salted water to the boil.
Prep potatoes and cook pot of boiling water for 12 minutes, until just beginning to soften. Drain well and allow potatoes to dry out in a colander. Meanwhile, prep onion, asparagus and lettuce and set aside.
Transfer dried potatoes to a lined oven tray, smash edges slightly with a fork, then drizzle with oil and season with salt and pepper. Roast on upper oven rack for 25-30 minutes, until golden and crispy.
Toss onion and asparagus on a second lined oven tray with olives and olive oil. Season with salt and pepper. Roast on lower oven rack for 15-20 minutes, until tender. Once cooked, toss with artichokes and lettuce.
When potatoes have about 5 minutes cook time remaining, cook fish. Heat oil in a medium, non-stick fry-pan on medium-high heat. Cook fish for about 2-3 minutes each side (depending on thickness), until golden and just cooked. Place fish on a plate and keep warm. Keep pan on heat.
Add butter to pan and cook for 1-2 minutes, until bubbling, browned and smelling nutty. Add capers and cook a further 1 minute. Remove from heat and stir through lemon juice.
Serve potatoes with niçoise, fish and caper butter.