Position a rack in the center of the oven and heat the oven to 400°F.
Heat a 3-quart saucepan over medium heat. Add the coriander and stir until fragrant, about 30 seconds. Add 3 cups of water, the lemon zest, and 1 tsp. salt; turn the heat up to medium high. Whisk in the cornmeal and cook, stirring often, until thick and creamy, about 20 minutes. (If the polenta gets too thick, loosen with a little hot water.)
Meanwhile, melt the butter in a small saucepan. Season the fish lightly with salt and pepper and arrange in a 9x13 baking dish with space between each piece. Remove the butter from the heat, stir in the harissa and lemon juice, and pour it over the fish. Bake, basting every 5 minutes or so, until cooked through (the fish will flake easily), about 15 minutes. If necessary, cover with foil to keep warm.
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