Whisk together stock, sherry, soy sauce, oyster sauce, cornstarch, oil and sugar in a small bowl; set aside.
In a bowl, toss together pork, cornstarch, sherry, salt, pepper and egg white. Let sit for 30 minutes and then drain.
Pour oil to a depth of 2 inches in a 14 inch wok or high-sided skillet over medium-high heat. Heat oil until a deep-fry thermometer reads 300 degrees F.
Add pork and fry, stirring gently constantly, until it turns white (45 seconds). Using a slotted spoon, transfer pork to a large colander set over a bowl to drain.
Heat oil to 350 degrees F. Add shiitakes, water chestnuts and both peppers to oil and fry, stirring often, until crisp-tender (30 seconds). Using a slotted spoon, transfer to colander with pork. Set aside.
Discard all but 1 tbsp oil and return wok to high heat. Add garlic and ginger and cook, stirring, until fragrant (10 seconds). Add sauce and cook, stirring, until thickened (20 seconds).
Return pork and vegetables to wok and toss until well coated. Transfer to a serving platter and serve with rice.