In a small bowl, whisk together the tamari, gochujang, minced ginger, and minced garlic to create the stir-fry sauce.
Season the diced chicken breast evenly with sea salt and black pepper.
Heat the toasted sesame oil in a large skillet or wok over medium-high heat until shimmering.
Add the chicken to the skillet and sauté for 5-6 minutes until golden brown and cooked through.
Add the sliced red bell pepper, snap peas, and chopped baby bok choy to the pan, stir-frying for 2-3 minutes until the vegetables are tender-crisp.
Toss the cooked udon noodles into the skillet and pour the prepared spicy sauce over the mixture.
Stir-fry for an additional 2 minutes, ensuring the noodles and vegetables are thoroughly coated and the sauce has thickened slightly.
Garnish with sliced green onions and sesame seeds before serving hot.