Place the shrimps between stacks of paper towels and press down lightly to remove excess moisture.
Cut the pechay into one-inch slices; separate the stalks from the leafy portions.
Heat the cooking oil in a pan.
Saute the shallot, tomatoes, garlic and ginger with half of the fish sauce and half of the pepper until the vegetables are just starting to soften.
Add the pechay stalks (they take longer to cook than the shrimps) and continue sauteeing for a minute.
Spread the shrimps in the pan and cook without moving them for a minute.
Drizzle in the rest of the fish sauce, sprinkle in the rest of the pepper and all of the sugar.
Stir the contents of the pan and cook until the shrimps have almost completely changed color.
Add the pechay leaves to the shrimps. Cook, tossing, until the leaves are wilted.
Taste and adjust the seasonings, if needed.
Serve your shrimps and pechay guisado immediately.