Preheat the oven to 350F.
Rinse and pat the chicken dry with a kitchen towel.
In a bowl, mix together the garlic, ginger, chilies, oyster sauce, salt, sugar and sesame seed oil. Add the chicken pieces and mix well.
Take two large sheets of foil. Arrange the chicken in a single layer on one side of one sheet, fold the foil to cover the chicken and fold all the open sides to seal the package. Place the package in the remaining sheet of foil and repeat the sealing process. The chicken will be cooking in its own juices and steam and no other moisture so you don’t want any of it escaping so that the chicken does not dry out.
Place the package in a baking tray and bake at 350F for 30 to 40 minutes, depending on the size of the chicken portions. In calculating the time, remember too that chicken fillets will cook much faster than pieces with bones. While the chicken bakes, read a book, watch the TV, take a bath or whatever pleases you.
When cooking time is up, take the tray out of the oven. Leave the chicken inside the package to cool a bit.
Time to cook the vegetable side dish.
Cut the swamp spinach into two-inches lengths, separating the stalks from the leafy portion.
Heat the sesame seed oil in a wok or frying pan. Add the swamp spinach stalks to the hot oil first as they need to cook longer than the leaves. Season lightly with salt and pepper. Cook, stirring often, for a minute or so. Then, add the leaves and the garlic. Sprinkle with more salt and pepper. Stir fry for another minute or so or just until the leaves start to go limp.
Cut the foil open — carefully so that the juices don’t spill out. Take out the chicken and arrange on one side of a platter. Fill the other side of the platter with the garlic spinach. Drizzle the chicken juices and spoon the aromatics that remain inside the folded foil over the chicken.
Serve the baked spicy chicken with garlic spinach at once.