Preheat the oven to 180C/350F/Gas 4.
For the potato-crusted haddock, grate the potato into a clean tea towel or cloth and squeeze out the excess moisture.
Season the haddock fillet with salt and freshly ground black pepper, press the grated potato onto one side of the fillet and season again with salt and freshly ground black pepper.
Heat the olive oil in an ovenproof frying pan, add the haddock, potato side down, and fry for 3-4 minutes, or until golden brown. Carefully turn the fish over, then add the butter and place the pan in the oven to roast for 3-4 minutes, or until the fish is just cooked through and the topping is golden-brown and crisp.
For the butter sauce, melt the butter in a saucepan over a medium heat, add the onion and lightly fry for 4-5 minutes, or until softened but not coloured. Pour in the wine, turn up the heat and boil for five minutes, or until the liquid has reduced by half. Stir in the cream, season with salt and freshly ground black pepper, then reduce the heat and simmer for 3-4 minutes, or until the sauce has thickened slightly.
For the broccoli, cook the broccoli in boiling salted water for 5-6 minutes, or until tender. Drain well. Heat a small wok or frying pan until smoking, add the broccoli and drizzle over the soy sauce and sesame oil. Stir to coat, then sprinkle with the sesame seeds.
To serve, pile the broccoli onto a serving plate, top with the haddock and drizzle over the butter sauce.