Ask your local butcher to debone the duck for you.
In a medium-size bowl, combine sugar, soy sauce, and egg. Add duck and toss to combine. Cover and refrigerate 4-6 hours (or overnight).
Remove duck and discard marinade. Place marinated duck and 1 tablespoon cornstarch in a large zip-lock plastic bag and shake to coat.
In a wok or large, heavy-bottomed sauté pan, heat oil on high setting until it begins to smoke. Add duck and cook, stirring frequently, until duck is well-browned on all sides, 6-8 minutes. Using a slotted spoon, transfer duck to plate.
Reduce heat to medium-high. Add onions and sauté until softened, about 5 minutes. Add garlic and ginger and cook until fragrant, about 30 seconds.
In a small bowl, combine brown sugar and vinegar. Add to pan, stirring well.
Add tomato, peas, and pineapple.
Return duck to pan.
In small bowl, mix remaining cornstarch and sherry and stir into pan.
Simmer until mixture thickens slightly, about 2 minutes. Serve over rice.