2 stalks lemongrass, white part only, sliced diagonally
2 teaspoons salt
Place the ginger, garlic, lemongrass, vinegar, calamansi juice, salt, soy sauce, brown sugar, annatto oil, and black pepper in a bowl and mix thoroughly. Add the pork and toss to coat. Marinade for 4-6 hours to overnight (but no longer!).
Make the annatto oil: place the oil, annatto, garlic, and lemongrass in a medium skillet and put over low heat. Once bubbles start to appear around the aromatics, give it a stir and remove from the heat. Let this steep for about 5 minutes off the heat then strain. Add salt to the strained oil and stir.
When ready to cook, remove pork from marinade, making sure to brush off bits of lemongrass and garlic as these will burn. Bring the grill or a grill pan to high heat and brush with oil. Place the pork belly slices on the grill and cook for two minutes. Flip slices and brush the cooked sides with annatto oil, and cook for another two minutes. Flip again and brush the other side with annatto oil, and cook until done.