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.