Wash and dry all produce. Halve, peel, and thinly slice onion. Halve, core, and thinly slice bell pepper into strips. Peel and mince ginger. Trim and thinly slice scallions, separating whites from greens. Quarter lime.

In a small bowl, combine scallion whites, juice from half the lime, 1/2 tsp sugar (1 tsp for 4 servings), and a pinch of salt. Stir until sugar and salt have mostly dissolved. Set aside, stirring occasionally, until ready to serve.

Heat a drizzle of oil in a large pan over medium-high heat. Add beef and ginger; season with salt and pepper. Cook, breaking up meat into pieces, until browned and cooked through, 4-6 minutes. Turn off heat. Using a slotted spoon, transfer beef to a plate and set aside. Wipe out pan.

Heat a drizzle of oil in pan used for beef over medium-high heat. Add bell pepper and onion; season with salt and pepper. Cook, stirring, until softened, 5-7 minutes. Return beef to pan and add teriyaki sauce. Cook, stirring, until mixture is coated and sauce is slightly thickened, 1-2 minutes. Turn off heat.

Meanwhile, wrap tortillas in damp paper towels and microwave until warm and pliable, 30 seconds. In a second small bowl, combine mayonnaise, ponzu, and sriracha to taste.

Divide beef filling and pickled scallion whites between tortillas. Drizzle with sriracha-ponzu mayo and top with scallion greens. Serve with remaining lime wedges on the side.