Cut and discard ends of upo. Using a peeler, peel skin. Cut the gourd lengthwise.
If the interior is "too spongy" and has mature seeds, remove spongy flesh using a spoon and discard. Slice gourd lengthwise into halves and then slice into 1/2 inch-thick pieces.
In a pot, heat oil. Add onions and garlic and saute until softened.
Add tomatoes and continue to cook until slightly softened.
Add fish sauce and cook, stirring occasionally, for about 1 to 2 minutes.
Add sardines and gently break into pieces, if desired.
Add water and bring to a boil.
Add upo and stir to combine. Continue to cook for about 5 to 7 minutes or until upo are tender yet crisp.
Season with salt and pepper. Serve hot.
Notes
Cook the vegetable just until tender as it will continue to cook and soften in the residual heat.