In a pan, heat about 2 tbsp cooking oil from canned tuna. Add garlic and onion sauté until fragrant.
Add potato, carrots, water, and remaining oil from canned tuna. Simmer for 2 minutes.
Add sugar, pepper, raisins, green peas, and tuna flakes. Cook while occasionally stirring until all liquid dried up. Remove from heat then set aside.
In a mixing bowl, combine flour, baking powder, and salt. Mix thoroughly.
Add cold butter pieces. Knead with your clean hands until the butter is fully incorporated.
Gradually add cold water while kneading until a soft dough is formed. Cover with plastic then chill for 30 minutes.
Spread some flour to your flat workspace.
Divide the dough into 6 to 8 portions. Flatten each portion using a rolling pin.
Place about 2 tbsp of the filling on the center of one dough portion. Add some grated cheese.
Fold and press the edges of the dough on top of each other. Seal it off by using a fork. Repeat steps 9-10 for the remaining portions and fillings.
Deep-fry the empanadas by batch until golden brown. Remove from heat then drain excess oil.
Serve and enjoy your cheesy tuna empanada!