Preheat the oven to 200°C/180°C fan/400°F/Gas mark 6.
Put the potatoes and oil in a baking tray and toss to combine. Season well with salt and pepper and bake for 15 minutes until slightly golden. Add the onion, tomatoes, rosemary and balsamic vinegar, combine, and bake for another 10 minutes.
Season the salmon fillets and add to the tray of vegetables together with the asparagus and cook for a final 10-12 minutes, until the salmon is just cooked. Serve immediately.