In a large pot over medium heat, combine the black peppercorns, coriander, fenugreek or white peppercorns, and cumin, and toast for 30 seconds or until fragrant but not burnt. Grind the spices with a mortar and pestle or spice grinder and set aside.
In the same pot over medium heat, add neutral oil and let heat for 1 minute. Add the onion, ginger, and garlic, and cook, stirring occasionally, until tender but not brown, about 4 minutes. Add the ground toasted spices, plus the turmeric, cinnamon, salt, tomato paste, and the stick of butter and stir. Add the tomatoes and jalapeños.
Cook, stirring occasionally, until everything is fully soft, 15–20 minutes. Take off the heat. Stir in the crème fraîche. Let cool a bit, then blend in batches in a blender. Return to pot. Adjust salt if needed. Set aside.
Char the corn on a grill or in a cast-iron pan. Cut the corn off the cob, set aside, and reserve the cobs.
Cover the cobs with 4 cups water in a medium pot. Bring to a boil, then simmer for 30 minutes. Strain the liquid through a fine mesh strainer, reserve, then discard the cobs.
Return the charred corn kernels to the pot, and cover with the corn broth. Cook until tender. Season with salt, pepper, and remaining 2 tbsp. butter. Keep warm in pot for dish assembly.
For the cod: Preheat oven to 400°F. Cut the cod into four equal pieces. Remove any bones. Set the cod in a parchment-lined baking dish. Sprinkle with salt and a drizzle of olive oil.
When you are ready to serve the dish, put in the oven. Roast the cod in the oven until fully cooked and tender, 7–10 minutes. The cod should flake when squeezed, indicating it is fully cooked. You can also check for doneness by inserting a metal skewer into the center or thickest part of the fish. There should be no resistance, and the skewer should be hot to the touch from the fish.
To assemble the dish, rewarm the tomato curry. Set the cooked pieces of fish into individual serving bowls. Spoon the corn kernels on top of the fish. Scoop the tomato curry on top of the corn and fish. Drizzle olive oil to finish.