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Combine 1 Tbsp soy sauce and Sherry in a bowl, then toss with shrimp.
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Stir together 1 cup water, remaining 2 Tbsp soy sauce, sugar, vinegar, and 1/4 tsp salt in another bowl until sugar has dissolved. Reserve vinegar sauce.
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Put 1/4 cup cornstarch in a shallow bowl. Drain shrimp (discard marinade), then dredge in cornstarch, knocking off excess, and transfer to a plate.
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Heat 1/3 cup oil in a wok or 12-inch heavy skillet over medium-high heat 2 minutes. Stir-fry shrimp in 2 batches until golden and just cooked through, about 2 minutes per batch, transferring to a clean large plate as cooked.
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Stir-fry onion and bell peppers 3 minutes, then transfer to plate with shrimp.
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Add 1 Tbsp oil to wok, then stir-fry carrot and pineapple 2 minutes. Transfer to plate. Add remaining Tbsp oil and stir-fry scallions, garlic, and ginger 1 minute.
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Add vinegar sauce to wok and bring to a brisk simmer. Whisk remaining 2 tsp cornstarch into remaining 2 Tbsp water, then add to wok and boil 1 minute (sauce will thicken). Return shrimp, vegetables, and pineapple to wok and stir-fry until heated through and coated with sauce, 1 to 2 minutes.