To begin, rinse the rice thoroughly under cold water until the water runs clear. Soak the rice in water for at least 4 hours or overnight. This softens the grains and allows for even steaming. Drain before cooking.
Set up a steamer lined with cheesecloth or muslin. Steam the soaked rice for 25–30 minutes over high heat. Halfway through, fluff the rice gently with a fork to ensure even cooking. Once translucent and tender, remove from the steamer.
While the rice is still warm, transfer it to a large mixing bowl. Gently fold in coconut milk, fenugreek seeds, sliced shallots, salt, and the knotted pandan leaf. Let the rice rest covered for 15 minutes to absorb the flavours and finish cooking in its residual heat.
Prepare the spice paste by blending all the paste ingredients into a smooth mixture. Add a splash of oil or water if needed for blending. This paste forms the heart of the curry, so ensure it's well-blended.
Heat 2 tablespoons of oil in a pot over medium heat. Sauté the spice paste for 5–7 minutes until fragrant and the oil begins to separate. Stir continuously to avoid burning and coax out the deep aroma of the spices.
Add the bruised lemongrass, tamarind water, and asam keping to the sautéed paste. Simmer for 3–4 minutes, allowing the tartness to blend into the spice base. Taste and adjust with a pinch of salt or sugar as needed.
Pour in the coconut milk and bring the curry base to a gentle simmer. Stir occasionally and cook uncovered for about 10 minutes. Avoid boiling to prevent the coconut milk from splitting.
Add the tuna steaks into the simmering curry. Cook gently over low heat for 15–20 minutes or until the fish is cooked through and tender. Spoon the curry over the fish occasionally to baste it in the sauce.
Once cooked, remove from heat and let the curry rest for 10 minutes. This allows the flavours to deepen and the oil to rise to the top, which is characteristic of a well-balanced gulai.
To serve, spoon the steamed coconut rice onto individual plates. Place a generous piece of the fish on the side and ladle over the rich curry sauce. Garnish with fresh cucumber slices or pickled vegetables (acar) for contrast. Serve warm, ideally on a banana leaf for a traditional touch.