Wash the chicken to remove any impurities. Put in a colander and let it drain until dry.
Rub with calamansi juice, salt and pepper. Let it stand for a few minutes.
In a large pot that will fit the whole chicken, put the orange soda softdrink (Royal tru orange), onions, ginger, lemongrass, leeks, spring onions, garlic, oyster sauce, soy sauce and ketchup.
Then lay the whole chicken breast side up to avoid over cooking the breast part. Cover and simmer it for 30 to 45 minutes.
After 30 minutes, open the pot and turn the chicken upside down facing the breast part on the bottom of the pot. Simmer again for 15 minutes.
Then turn off heat and remove the chicken from the pot. Remove all the aromatics in the pot and leave the sauce. You can also strain the aromatics using a sieve and put the liquid back in the pot.
Put the chicken back in the pot and turn on the heat. Baste the chicken with the sauce from time to time while cooking.
Cook until the liquid becomes thick. Serve hot and garnish with sliced leeks.