Place the diced potato in a small pot. Cover with water and add in 1 heaping teaspoon salt. Bring the water to a boil then reduce the heat to low and simmer for a 3 to 5 minutes until a fork can be inserted into the potato easily, without the potato disintegrating. Drain the potato into a colander and set it aside to drain the excess water (no need to rinse the potato).
Heat a few tablespoons of oil in a frying pan. Fry the onion over medium to medium-low heat until onion has became translucent. Add in turmeric and continue frying until fragrant, a couple of minutes more. Take care not to burn the mixture.
Drain the oil out of the chilli tuna and remove the chilli. Add into the frying pan together with the potatoes and continue to cook for a few more minutes, adding more oil if necessary. Add in sugar and season with salt to taste. If it's still too spicy, add in more sugar. Remove from heat and set aside to cool.
The filling can be made in a day in advance - store in an air-tight container in the refrigerator. Bring to room temperature before using.
Dough
Measure the flour, sugar and salt in a mixing bowl. Add in water and margarine and knead until the mixture is smooth and the dough no longer sticks to your fingers, about 10 minutes. Cover the mixing bowl with a damp kitchen towel and set aside for the dough to rest for 10 minutes.
After resting for 10 minutes, the dough should be soft, elastic and very easy to handle. Pinch a small piece of dough, roll it into a ball and flatten into a circle with a rolling pin. Make it as thin as possible without tearing the dough. Scoop the filling into the centre and fold the dough into a semi circle. Pinch the edges to seal then crimp the ends to further seal in the filling.
Heat oil in a small pot, about 1 to 1.5-inch deep over medium to medium-low heat. When the oil is hot enough, slide in the puffs gently and deep-fry until the puffs are golden brown in colour.
Serve hot or warm. Do not leave the puffs out for too long as the skin will lose its crispiness.