Thinly slice the chicken breast into bite-sized strips and season with sea salt and black pepper.
Wash the baby bok choy thoroughly, separating the white stems from the green leaves; roughly chop both parts.
Heat the avocado oil in a large wok or cast-iron skillet over medium-high heat until shimmering.
Add the chicken to the pan in a single layer and sear for 3-4 minutes until golden brown and cooked through, then remove chicken and set aside.
In the same pan, add the sliced shiitake mushrooms and the white stems of the bok choy, sautéing for 2 minutes until they begin to soften.
Stir in the minced garlic, grated fresh ginger, and red pepper flakes, cooking for 30 seconds until highly aromatic.
Return the chicken to the pan along with the green bok choy leaves, tamari, and toasted sesame oil.
Toss everything together for 1-2 minutes until the leaves are wilted and the sauce coats every ingredient.
Garnish with thinly sliced green onions and serve immediately while hot.