Heat a large skillet or wok over medium-high heat until hot.
Add 1 1/2 tablespoons unsalted butter and 1 1/2 tablespoons vegetable or olive oil. When the butter melts and begins to froth, add 2 teaspoons garlic, minced, and 1 small yellow onion, roughly chopped. Stir constantly and cook about 2 minutes, until the onion softens.
Add 1 zucchini cut into strips, 1 cup sliced carrots, 2 cups broccoli florets, and 6 ounces mushrooms, quartered. Stir to combine and cook about 1 minute.
Pour in 1 tablespoon soy sauce and 2 tablespoons steriyaki sauce. Season with salt and pepper to taste. Continue to cook, stirring frequently, for about 10 minutes total, until the vegetables are tender-crisp.
Remove from heat and transfer the vegetables to a serving dish.
If using, sprinkle sesame seeds on top. Serve hot.