Preheat the oven to 200°C.
Peel the onion and garlic. Slice the onion and crush the garlic and place into a roasting tin.
Add the halved cherry tomatoes. Slice the fennel and add to the tin. Season well and toss in the olive oil.
Roast the vegetables for 30 minutes until lightly browned.
Remove from the oven and stir in the chickpeas and mussels. Place the trout fillets on top (skin side up) and return to the oven for a further 10 minutes until the trout is just cooked through and the skin crisp.
Carefully remove the trout fillets to serving plates and sprinkle the herbs over the salad. Lightly mix together and serve immediately with the trout.