Heat the oven to 200C/180C fan/gas 6 with a large roasting tin filled with the breadcrumbs inside. When golden (about 5 mins), tip into a bowl and set aside. Drizzle 2 tbsp of the oil into the tin, add the potatoes, season and toss to coat. Spread them out in an even layer and roast for 25 mins.
Meanwhile, mix the breadcrumbs with the anchovies, herbs and garlic granules. Toss the broccoli with the remaining oil and season.
Once the potatoes have had 25mins, scatter over the broccoli. Cook for a further 5 mins, then nestle in the fish. Top with the breadcrumbs. Return to the oven for 8-10 mins until the fish is cooked through and golden. Serve with tartare sauce, if you like.