01 - In a small bowl, whisk together the soy sauce, oyster sauce, mirin, sesame oil, and sugar until the sugar dissolves. Set aside.
02 - Prepare the udon noodles according to the package directions. If using frozen or dried noodles, boil until tender. Drain well and set aside.
03 - Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the shrimp and stir-fry for 2 to 3 minutes until they turn pink and opaque. Remove the shrimp from the pan and set aside.
04 - Add the remaining tablespoon of vegetable oil to the same pan. Sauté the minced garlic and grated ginger for about 30 seconds until fragrant.
05 - Add the sliced onion, julienned carrot, and sliced bell pepper to the pan. Stir-fry for 3 to 4 minutes until the vegetables are just tender but still have a slight crunch.
06 - Return the cooked shrimp to the pan and add the drained udon noodles. Pour the prepared sauce over everything and toss thoroughly to coat the noodles and vegetables evenly.
07 - Add the baby spinach or bok choy along with the sliced green onions. Stir-fry for 1 to 2 minutes until the greens are just wilted.
08 - Transfer to plates or bowls and serve immediately, garnished with toasted sesame seeds, shredded nori, and lime wedges if desired.