Preheat the oven to gas 7, 220°C, fan 200°C. Cut the top off the garlic bulb to expose the ends of the cloves. Drizzle with 1 tsp oil, wrap in foil and roast for 40 mins until really soft.
Meanwhile, season the salmon on both sides. Melt the butter in a large, flameproof heavy roasting tin (big enough for the salmon), add the capers and half the roughly chopped herbs.
Put the salmon in the tin, skin-side down, and baste the top with the herb butter. Add the orange slices, tucking some underneath. Roast for 15-20 mins until the thickest part of the fish is cooked through.
For the herb cream, squeeze the soft garlic from its skin into a bowl and mash with the remaining 1 tbsp oil, the mustard and lemon juice; season. Stir in the crème fraîche and finely chopped herbs.
Transfer the salmon to a warm serving platter and drizzle any herby butter and capers from the tin over the top. Decorate with the roasted orange slices and the parsley and dill leaves. Serve with the herb cream.
