Peel the upo and cut in half lengthwise. Remove hard seeds; leave soft seeds in. Cut into bite-sized pieces, about ¼ inch thick.
Mix soy sauce, oyster sauce, sugar, and water in a bowl.
Heat a little oil in a wok or large pan over medium-high heat. Lightly salt the shrimp and sear until pink and opaque, about 2 minutes total. Transfer to a plate.
Lower the heat to medium. Cook garlic and onion until softened, about 2 minutes. Add tomatoes and cook until soft and starting to caramelize, about 5 minutes. Add optional chilies and cook for a minute.
Add upo and sauce. Cook for 3–5 minutes, stirring occasionally, until tender. Add water only if needed; the upo releases liquid as it cooks.
Return shrimp and heat through. Season with salt and pepper, then turn off the heat.
Notes
Upo: Choose firm ones without soft spots. Peel and remove hard seeds; soft seeds can stay. Substitute zucchini or sayote.
Shrimp: Use peeled and deveined shrimp, or substitute chicken, pork, tofu, or fish.
Tomatoes: Use ripe tomatoes; cook until soft and starting to caramelize.
Oyster sauce: Substitute fish sauce, adding a little at a time to taste.