Slice the pork into scallops, about 1.5 x 0.125 inches thick.

Set the pork aside.

In a small bowl, combine 1/4 cup water, nam pla (fish sauce), sugar, and ketchup.

Set the sauce mixture aside.

In another small bowl, mix tapioca starch with 2.5 tablespoons of water.

Reserve the tapioca starch slurry.

Heat a wok over high heat for 30 seconds.

Add peanut oil to the wok and wait until a wisp of white smoke appears.

Add minced garlic to the hot oil.

Cook the garlic until it turns light brown.

Add the sliced pork to the wok.

Spread the pork in a thin layer in the wok.

Cook the pork for about one minute.

Turn the pork over and cook until the pink color vanishes, approximately one minute more.

Add sliced onions and cubed tomatoes to the wok.

Stir the onions and tomatoes with the pork for one minute.

Add white vinegar to the wok.

Mix all the ingredients well.

Pour the nam pla mixture into the wok.

Stir well to combine all the ingredients.

Add scallions, green bell peppers, minced red serrano chili, and cucumber to the wok.

Mix thoroughly.

Stir the tapioca starch mixture again and add it to the wok.

Stir continuously, combining the ingredients, until the sauce bubbles and thickens.

Turn off the heat.

Transfer the Pad Priew Wan to a heated serving dish.

Serve hot with plain cooked rice.