Season the salmon fillets generously with salt and pepper and pour over the lemon juice. Set aside.
Combine the oil, vinegar, honey, garlic and salt and pepper to taste in a small bowl. Put the onions, fennel and tomatoes in a large bowl and pour over the oil mixture. Toss to coat thoroughly, then spread out on a baking sheet.
Roast in a preheated oven, 220°C (425°F), Gas Mark 7, for 10 minutes. Add the salmon fillets to the baking sheet and roast for a further 12–15 minutes.
Serve the salmon with the roasted vegetables and rice or couscous.