In a large pan over medium-high heat, saute the ground pork until slightly browned. Season with salt & pepper. Next add the sliced onions and continue to cook until onions are slightly translucent. To that, add the chili-garlic paste, oyster sauce, soy sauce, brown sugar, sesame oil & fish sauce. Combine ingredients well and bring contents to a simmer. Have your cornstarch slurry ready for thickening the sauce. Cornstarch tends to settle even when mixed with water, so make sure the slurry is homogeneous before you add it to anything you thicken. In other words, pre-stir your slurry before adding it to any heated liquid, or you may have an inconsistent thickening agent, or it may also clump. Next, add the water to the contents of the pan and bring back to a simmer. Now add in the cornstarch slurry quickly and evenly. If you can mix and add the slurry together at the same time, you will have a nice evenly thickened sauce. The thickened mixture needs to come back to a simmer for a few minutes to ensure the starches have been absorbed to fully thicken the sauce. Otherwise it will be runny and taste chalky. To the thickened sauce, add in the cubed tofu and fold in gently to heat through. Also add in the green onions. Re-season with salt and pepper if necessary. Serve over hot rice and garnish with sprigs of Chinese parsley, green onions and sesame seeds.