Slice squash: Trim zucchini ends, cut lengthwise, then slice into 1/4″ half-moons. Spread on paper towels to absorb moisture.
Cut peppers: Remove seeds, dice into 1″ pieces. Rinse under cold water to remove bitterness.
Prepare onion: Thinly slice to maximize surface area for caramelization.
Heat wok: Over medium-high heat, add oil until shimmering. Gently brush oil up the sides for even heating.
Scorch garlic: Add garlic and chili flakes, stir 15-20 seconds until fragrant but not browned.
Color onions: Add onions, stir fry 2 minutes until lightly golden.
Add zucchini and bell peppers to the pan; stir fry for 3-4 minutes, avoiding overcooking.
Porous soy sauce to the pan, tossing to coat vegetables.
In a small bowl, mix cornstarch with 2 tbsp water to form a slurry and add to the pan.
Cook and stir until the sauce thickens and gathers around the vegetables.
Remove from heat, stir in sesame oil.
Serve immediately, ideally over steamed jasmine or basmati rice.