Preheat the oven to Gas Mark 6, 200°C (400°F)
Spread the potatoes and onion over the base of a large roasting tin. Drizzle with 1 tablespoon oil, season then toss to coat. Cook in the preheated oven for 15 minutes. Add in the tomatoes and cook for another 10 minutes.
Reduce the oven to Gas Mark 4, 180°C (350°F). Season the hake fillets and add them to the roasting tin. Grate the rind off the lemon and reserve for the dressing. Then cut 4 thin slices off the lemon and place a slice on top of each fillet. Keep the juice from the rest of the lemon for the dressing. Scatter the sliced chilli over the fish. Return to the oven and cook for 10-12 minutes until the fish flakes easily with a fork.
Meanwhile mix the garlic, lemon rind and juice, parsley and remaining 2 tablespoons of olive oil in a food processor to make the herb dressing. Taste and season.
Remove the roasting tin from the oven, top with the herb dressing and serve with steamed broccoli.