In a non-reactive medium bowl, combine the shrimp, mirin, soy sauce, scallion, and red pepper flakes. Marinate in the refrigerator for at least 1 hour and up to 6 hours.

Heat a 12-inch skillet over medium-high heat. Add the sesame oil and then the garlic and ginger, and cook, stirring, until the garlic begins to color, about 10 seconds. Add the beans and stir quickly to coat with the garlic and ginger. Continue to cook, stirring, until the beans start to turn bright green, 1 to 2 minutes. (If the garlic starts to burn, remove the pan from the heat and continue to stir quickly.)

Add the shrimp and the marinade (the liquid hitting the hot pan will create steam, so be careful). Cook, stirring constantly, until the juices have reduced and thickened slightly and the shrimp are pink and curled, an additional 3 to 4 minutes. Serve immediately.