Trim about 2 inches from the kangkong stalks and discard. Cut kangkong into 3-inch lengths, separating the sturdier stalks from the leaves. Wash thoroughly and drain well.
Drain tofu from packing liquid. Pat dry with paper towels to absorb moisture. Cut into 1/2- inch cubes.
In a bowl, combine oyster sauce, chili garlic sauce, water, sesame oil and cornstarch. Stir until well-blended and set aside.
In a wide skillet over medium heat, heat oil until very hot. Add tofu and spread into a single layer. When starting to crisp, turn and continue to cook until lightly browned. Remove from pan, drain excess oil on paper towels and keep warm.
In the pan, add onions and cook until limp. Increase heat to high. Add oyster sauce mixture and cook for about 1 minute or until bubbly and begins to thicken. Add tofu and sturdier stalks of kangkong and cook, stirring regularly, for about 1 minute. Add kangkong leaves and cook, stirring regularly, for another 1 to 2 minutes or until tofu is heated through and vegetable is tender yet crisp. Season with salt and pepper to taste. Serve hot.