Toss the shredded chicken with the lemon juice, soy sauce, olive oil, ginger, garlic, and pepper. Cover and refrigerate for 20 minutes, no longer than 1 hour.
Bring the broth to a bare simmer in a saucepan and keep it hot over low heat.
In a wide, heavy pot, melt 1 tablespoon butter with the oil over medium heat. Cook the onion for 4 to 5 minutes, until soft and translucent but not browned. Add the garlic and stir for 30 seconds.
Add the rice and stir for 2 minutes, until the edges look translucent and the grains smell nutty, with a white dot still visible in the center.
Add 1 cup (240ml) hot broth and stir until it is mostly absorbed. Continue adding broth a ladle at a time, stirring often, and waiting until the pot looks almost dry on the bottom when you drag the spoon through, before each addition.
After about 10 minutes, stir in the bok choy stalks. Keep adding broth and stirring for another 8 to 10 minutes, until the rice is creamy and tender with a slight firmness at the center when bitten.
Stir in the bok choy leaves and the marinated chicken with its juices. Cook for 1 to 2 minutes, until the leaves turn bright green and the chicken is hot at 74°C (165°F).
Remove from the heat. Stir in the remaining butter, Parmesan, and lemon juice. The risotto should flow slowly and settle into a soft wave when the pot is tilted. Add a splash of broth if it is too thick, then season with salt and pepper and top with scallions.