For the crust mixture: Combine the flour, cracker crumbs, chili powder, salt and pepper in a wide shallow bowl.
For the tomato butter: In a large skillet over high heat, char the tomatoes on all sides until skins are black and begin to blister. Remove from pan and transfer to sauce pan. In the skillet, brown the onions with olive oil. Add the onions, shrimp stock, and sugar to the sauce pan and bring to a simmer over medium heat. Cook and reduce by half. Add heavy cream and reduce by half again. Whisk in butter, one tablespoon at a time, up to 6 tbsp (reserve 2 for later). Transfer to a blender and puree until smooth. Return to pan, season with salt and pepper to taste, and keep warm (careful not to break the sauce). Recommend warming up just before fish is ready.
Preheat over to 400 degrees.
Heat canola oil over high heat in a large ovenproof saute pan.
Dredge fish in crust mixture. Place fish in saute pan and cook for 2 minutes, until a crust forms on the fish. Turn fish over, add remaining 2 tablespoons of butter to the pan, and transfer to the oven. Bake for 3-6 minutes, or until fish is opaque throughout (baking time will depend on thickness of filets).
Serve fish with sauce on top. Garnish with chives.