Cut the tuna fillet into cubes then place in a food processor.
Add coconut flour into the food processor then process until smooth.
Transfer the tuna mixture to a bowl then add chopped leek to the bowl. Mix until just combined.
Shape the tuna mixture into small balls form then set aside.
Pour2 cups of water into a pot then bring to boil.
Once the water is boiled, slowly put the small tuna balls then cook until they are floating.
In the meantime, preheat a skillet over medium heat then pour extra virgin olive oil into the skillet.
Once the oil is hot, stir in minced garlic then sauté until aromatic.
Pour the remaining water into the skillet then bring to boil.
Once it is boiled, season the gravy with pepper then add chopped tomatoes and celeries to the gravy.
When the tuna balls are floating, take them out of the pot and transfer to the gravy. Bring to a simmer.
Transfer to a serving bowl then serve warm.
Enjoy immediately.