Slice the zucchini into 1/4-inch half-moons, cut the onion into 1-inch chunks, slice the mushrooms, and cut the red bell pepper into 1-inch pieces.

Whisk together soy sauce, mirin, sesame oil, white pepper, and salt in a small bowl and set aside.

Place a cast iron skillet or griddle over high heat for 3 minutes until it begins to smoke slightly.

Add the unsalted butter and vegetable oil to the hot pan and swirl to combine; the butter should foam immediately.

Add the yellow onion chunks and sliced mushrooms to the pan and let sit undisturbed for 2 minutes to develop color.

Stir the onions and mushrooms, push them to the outer edges of the pan, add zucchini and bell pepper to the center, cook for 1 minute, then stir everything together.

Continue cooking and stirring for 3 to 4 minutes until the vegetables are tender-crisp with visible browning.

Push all vegetables to the sides of the pan, add the minced garlic to the center, cook for 15 seconds until fragrant, then pour the sauce over everything and toss to coat.

Toss the vegetables and sauce together for 30 seconds until the sauce reduces slightly, transfer to a serving plate, and garnish with sesame seeds and sliced green onions.