Prepare all your ingredients: thinly slice the radishes and mince the garlic. Keep everything within reach for quick cooking.
Heat the vegetable oil in a large wok or skillet over medium-high heat until shimmering and hot, with a gentle sizzle when you add water.
Add the minced garlic to the hot oil; stir quickly and listen for the fragrant sizzle as it turns golden after about 15 seconds. Keep stirring to prevent burning.
Immediately toss in the sliced radishes, stirring vigorously to coat them evenly with the garlic and oil. You should hear a gentle sizzling sound as they hit the pan.
Cook the radishes for 2-3 minutes, stirring often, until they start to soften slightly and become translucent around the edges, with a fresh crunch still evident.
Add the soy sauce and chili flakes, stirring quickly to distribute the flavors evenly. Continue stir-frying for another 2-3 minutes until the radishes are tender but still hold their shape and a lively bite.
Remove the pan from heat and drizzle the toasted sesame oil over the radishes. Toss gently to combine, releasing a nutty aroma.
Transfer the stir-fried radishes to a serving plate. Garnish with sliced spring onions if using. Serve immediately while hot, crisp, and fragrant.