Gather a wok or large skillet, a sharp knife, and a cutting board. Slice the radishes into thin, even rounds about 2-3 mm thick to ensure quick cooking and crisp texture. Mince the garlic finely, releasing its fragrant aroma.
In a small bowl, combine soy sauce, a squeeze of lemon, and chili flakes if using. Set this flavorful sauce aside for later.
Heat the wok over high heat until it shimmers. Add the neutral oil, and once it starts to crackle softly, quickly add the sliced radishes.
Sauté the radishes for about 30 seconds, stirring constantly, until their edges start to turn golden and smell toasty.
Add the minced garlic to the pan, stirring rapidly for about 20 seconds until fragrant and just starting to turn golden, being careful not to burn.
Pour the prepared sauce over the radishes and garlic, stirring constantly. Cook for another 1-2 minutes until the radishes are tender but still crisp, and the sauce is glossy and coats everything evenly.
Remove the pan from heat and drizzle with sesame oil. Toss in the chopped green onions and give everything a quick stir to combine.
Squeeze a little lemon juice over the stir fry for a bright, fresh finish. Serve immediately while hot, garnished with extra green onions or chili flakes if desired.