Pat the tuna dry using paper towels. Then, lightly salt the tuna and set aside.
Heat coconut oil in a medium Dutch oven or a large pot over medium heat.
Add the red onions and saute until softened, about 6 - 8 minutes. Then, add garlic, ginger, curry leaves, pandan leaves, and scotch bonnet pepper. Cook until everything is fragrant, about 60 seconds.
Add the tomato paste to coat all the ingredients in the pot and cook until it is no longer vibrant red, about 2 minutes. Occasionally, stir to prevent the mixture from burning.
Add all the spices: crushed cardamom seeds, cumin, coriander, fennel, turmeric, chili powder, salt, and black pepper. Turn the heat to medium-low, mix well, and cook for 3 - 5 minutes, stirring frequently to let the spices enhance the flavors and prevent a gritty texture.
Pour the coconut milk and bring the curry to a boil for 5 minutes. Add the tuna and cook for another 5 minutes or until it's tender and opaque.
Season to taste with salt and pepper. Then, serve with basmati rice and roti. Enjoy!