Prepare the Noodles: If using frozen udon noodles, bring a pot of water to boil and cook the noodles according to package instructions. Fresh noodles can often be used directly in the stir fry. Drain and set aside once cooked.

Make the Sauce: In a small bowl, whisk together soy sauce, sugar, toasted sesame oil, fish sauce, lime juice, and white pepper until the sugar dissolves. Set aside.

Sauté the Vegetables: In a large skillet or wok, heat 2 tablespoons of neutral oil over medium-high heat. Add the garlic and Serrano chili, and stir-fry for about 1 minute until fragrant. Add the sliced shiitake mushrooms and stir-fry for 3-4 minutes, or until the mushrooms begin to soften.

Add Chinese Broccoli:

Toss in the Chinese broccoli and stir-fry for an additional 2-3 minutes, or until the greens are tender but still vibrant.

Combine Noodles and Sauce: Add the cooked udon noodles to the skillet. Coat the noodles and vegetables with the prepared sauce, making sure everything is evenly covered. Toss everything together until the noodles are well-coated, and cook for another 2-3 minutes until heated through.

Serve: Transfer the stir fry to serving plates or bowls. Garnish with extra sliced chili or a sprinkle of sesame seeds if desired, and enjoy while hot!