Heat the butter in a large pot over medium heat and add the onions, garlic, ginger and jalapenos. Sprinkle with salt and black pepper and sweat the vegetables until very soft and aromatic, stirring occasionally, about 8 minutes. Stir in the tomatoes, curry leaves, cumin and turmeric and simmer at least 10 minutes. Add the coconut milk and 1 cup water and continue simmering until all flavors have married and reduced to a coating consistency, about 2 hours.
Add 1/2 of the canola oil to a large saute pan over high heat. Sprinkle both sides of the halibut fillets with salt and black pepper. When the oil is screaming hot, add 1/2 of the halibut fillets and cook until golden brown, about 4 minutes, and then flip and cook another 4 minutes. Cook the remaining halibut in the remaining 1/8 cup oil.
To serve, spoon a bit of the curry sauce onto plates and place a halibut fillet on top. Sprinkle with a bit of the chervil, cilantro and mint.