Pat your shrimp dry with a paper towel. Heat 3 inches of oil in a small pot to 325°F. (Using a smaller pot conserves oil.) If you don’t have a thermometer, check the temperature by putting a drop of batter into the oil. The batter should sizzle, rise immediately to the surface, and stay a light color. If it turns brown right away, the oil is too hot.
To make the batter, mix together the flour, baking powder, cornstarch, salt, turmeric, and white pepper. Just before you’re ready to fry, mix in the sesame oil and cold seltzer water until the batter is smooth.
Next, drop the shrimp into the batter. Ensure they are evenly coated, but allow the excess to drip off. Carefully place the shrimp into the oil one piece at a time, ensuring that they don’t stick to each other. Fry in batches so the shrimp aren’t overcrowded and don’t stick together, turning the shrimp as needed and cooking each batch for about 2 minutes, or until golden brown.
Scoop the shrimp out using a slotted spoon, and transfer to a cooling rack placed over a baking sheet to drain. Repeat until all the shrimp has been fried.
Next, put two teaspoons of the frying oil in a separate wok or skillet over high heat, and toss in the onions and red and green bell peppers. Stir-fry for 30 seconds, then clear a place in the center of the wok. Add another teaspoon of oil, along with the ketchup, and fry for another 20 seconds.
Mix in the pineapple, pineapple juice, water, red wine vinegar, sugar, and salt. Bring the liquid to a low simmer for 2 minutes.
While the sauce is coming up to a simmer, refry your shrimp in batches (with the oil heated to 350°F). The temperature will drop when you add the shrimp. Continue frying at around 325°F for about 2 minutes.
With the sweet and sour sauce still simmering, slowly stir in the cornstarch slurry until the sauce is thick enough to coat a spoon.
Once all of the shrimp are fried, toss them into the wok, and fold into the sauce with 3 or 4 scooping motions until they are lightly coated. Serve immediately!