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Wash and dry produce.
Halve, peel, and dice onion into 1⁄2-inch pieces. Core, deseed, and dice bell pepper into 1⁄2-inch pieces. Thinly slice celery. Trim and thinly slice scallions, separating whites from greens. Dice tomato into 1⁄2-inch pieces.
Rinse shrimp* under cold water and pat dry with paper towels or open package of chicken* and drain off any excess liquid. Heat a drizzle of oil in a medium, heavy-bottomed pot over medium-high heat. Add shrimp or chicken in a single layer; season with salt and pepper. Cook, stirring occasionally, until cooked through, 4-6 minutes. Turn off heat; transfer to a plate. Wipe out pot.