1. Preheat the oven to 200°C, gas mark 6. Heat the butter with 1 tbsp olive oil in a large sauté pan. Add the leeks and cook gently for 5 minutes until softened but not coloured, then add the spinach and stir to wilt. Remove the pan from the heat.
2. Add the beans to the pan along with the crème fraîche, seasoned with freshly ground black pepper. Stir through the tarragon.
3. Divide the beans between 4 individual baking dishes and top with the smoked haddock. Combine the crust mixture with the Parmigiano Reggiano and scatter over the fish and beans.
4. Drizzle with the remaining olive oil, and bake in the oven for 25 minutes until golden and the fish begins to flake. Serve topped with a poached egg, if you wish.