Gather all your equipment: a large wok or skillet, a spatula, a small bowl, and measuring spoons, placing them within easy reach.
Prep your vegetables by slicing bell peppers and carrots into thin strips, and cutting broccoli into small florets. Keep them uniform in size for quick, even cooking.
In the small bowl, whisk together soy sauce, mirin, sesame oil, sugar, and water until the sugar dissolves completely. Set the sauce aside.
Heat your wok or skillet over medium-high heat until hot and slightly shimmering. Add a teaspoon of sesame or vegetable oil, swirling to coat the surface.
Add the minced garlic and ginger to the hot oil, sautéing for about 30 seconds until fragrant and aromatic.
Quickly toss in the sliced vegetables, stirring constantly. Cook for 2-3 minutes until they start to soften but still remain crisp and vibrant.
Pour the prepared sauce over the vegetables, stirring to coat evenly. Increase the heat slightly to bring the sauce to a gentle simmer.
Allow the sauce to bubble gently for 2-3 minutes, stirring often. The sauce should thicken slightly and turn glossy, clinging to the vegetables.
In the small bowl, dissolve the cornstarch in cold water to create a slurry, then pour it into the pan. Stir continuously until the sauce bubbles and becomes shiny and thickened, about 30 seconds.
Remove the skillet from heat once the sauce is glossy and coats each vegetable piece evenly. Sprinkle toasted sesame seeds over the top for added flavor and crunch.
Serve your vegetable teriyaki hot over steamed rice or noodles, garnished with extra sesame seeds if desired. Enjoy the vibrant colors, sticky glaze, and crisp-tender vegetables!