Prep vegetables: slice zucchini into half-moons and thinly slice the onion; mince the garlic.
Heat a large skillet or griddle over medium-high until hot.
Melt 1 tablespoon butter in the pan. Add onions and cook, stirring, until lightly golden, about 2 minutes.
Add zucchini in an even layer. Sear undisturbed 2 minutes to get some color.
Toss and season with salt, pepper, and sugar. Cook until crisp-tender, 2–3 minutes.
Stir in garlic and cook just until fragrant, about 30 seconds.
Pour in soy sauce and sesame oil. Toss until the vegetables are glazed, 1–2 minutes.
Cut heat, add remaining 1/2 tablespoon butter and lemon juice. Toss to melt, sprinkle sesame seeds, and serve hot.