Bring a large pot of salted water to a boil and blanch the trimmed green beans for 2 minutes.
Immediately transfer the beans to a bowl of ice water to stop cooking, then drain and pat very dry with paper towels.
Heat vegetable oil in a deep-fryer or large, heavy pot to 350°F (about 175°C) and maintain the temperature.
Set up three shallow bowls: one with the flour, one with the beaten egg, and one with the panko breadcrumbs.
Working in batches, coat a few green beans in flour, shake off excess, dip in the beaten egg, then press into panko to fully coat.
Carefully fry the coated green beans in the hot oil until golden brown, about 3–5 minutes per batch, turning as needed for even color.
Use a slotted spoon to transfer fried beans to paper towels to drain, then serve warm.
