Before you start, preheat oven to 220°C (or 200°C fan bake).
Prep onion. Toss pumpkin, onion and oil on a lined oven tray. Season with salt and pepper and roast for 25 minutes. Meanwhile, prep asparagus, lentils and carrot. After 25 minutes, remove tray from oven and add asparagus, balsamic vinegar and walnuts.
Return tray to oven to cook for a further 5 minutes, until vegetables are tender. Remove from oven and toss through lentils to warm through. Toss with carrots and baby spinach just before serving and season to taste with salt and pepper.
Pat fish dry and remove any remaining scales or bones. Cut larger fillets in half and season with salt.
When vegetables have 10 minutes cook time remaining, heat half the oil in a large fry-pan on medium-high heat. Cook fish in batches for about 2 minutes each side, until just cooked through. Use second half of oil for second batch.
To finish, prep basil and lemon.
Serve vegetables topped with fish and a dollop of lemon aioli. Sprinkle over basil and serve lemon wedges on the side for squeezing.