Preheat the oven to 200°C/400°F/gas 6.
Whisk half the olive oil and the dill into the grain mustard. Coat the top of the cod fillets with the mixture then place in a roasting tin.
Surround the fish with the tomatoes and drizzle with the remaining oil. Sprinkle with salt and pepper and roast for around 10 minutes, until the cod is just flaking and the tomatoes are starting to collapse.
Serve with boiled new potatoes tossed in a little butter and more dill.