Heat the wine in a large frying pan over medium heat and add 2 tablespoons of the butter. When the butter melts, stir to mix. Add the fish fillets in a single layer, adjust the heat to maintain a gentle simmer, and cook just until the fillets start to flake when prodded with a fork, 4-5 minutes, depending on their thickness. They will finish cooking in the oven. Carefully transfer the fillets to a plate. If you purchased fillets with the skin attached, let cool until they can be handled, then carefully remove the skin.

Reserve 1/4 cup of the poaching liquid for making the sauce and discard the remainder or save for another use.

To make the sauce, melt the butter in a saucepan, over medium-low heat. Stir in the flour and continue to stir, allowing the mixture to brown slightly without burning, for about 5 minutes. Whisk in the hot milk, little by little. When all the milk is added, whisk in the poaching liquid and continue to whisk until the sauce is smooth and creamy, 3-5 minutes. Add both cheeses and stir until melted. Season to taste with nutmeg and pepper and set aside off the heat.

Rinse the spinach and put in a large saucepan with just the rinsing water clinging to the leaves. Cover, set over medium heat, and cook until wilted and tender, about 5 minutes. Drain well, squeeze gently in a kitchen towel to remove the excess moisture, and chop roughly. Mix the spinach with the remaining 2 tablespoons butter and season with lots of pepper.

Preheat your broiler. Butter a broiler proof oven dish just large enough to accommodate the spinach and fish. Spoon the spinach into the dish, spreading evenly. Arrange the fillets in a single layer on top of the spinach. Pour the sauce over the top, then sprinkle evenly with the Parmesan. Broil until the cheese is browned and bubbling, 3- 5 minutes. Serve hot.