Cook the udon noodles according to package instructions until al dente, then drain and set aside.
In a small mixing bowl, whisk together the tamari, rice vinegar, and red pepper flakes to create the stir-fry sauce.
Heat the toasted sesame oil in a large skillet or wok over medium-high heat.
Add the shrimp to the skillet and sauté for 2-3 minutes until they turn pink and opaque, then remove them from the pan and set aside.
In the same skillet, add the minced garlic, grated ginger, sliced bell peppers, and chopped bok choy.
Stir-fry the vegetables for 3-4 minutes until the bok choy stems are tender but still retain a slight crunch.
Return the cooked shrimp and the udon noodles to the skillet, pouring the sauce over the mixture.
Toss everything together for 1-2 minutes until the noodles are well-coated and the entire dish is heated through.
Garnish with freshly sliced green onions and serve immediately.