Mix the cornstarch and water together in a bowl or sealable jar until smooth. Add the rest of the ingredients and stir or shake until fully combined. Set aside.
Heat 1 tablespoon of the avocado oil in a large skillet or wok over medium-high heat.
Once hot, add tofu and cook, flipping frequently, for about 10 minutes. Once all sides of the tofu cubes are golden, transfer to a plate and set aside.
Add the remaining tablespoon of avocado oil to the skillet with the onions and cook for 2 minutes until slightly softened.
Add the carrots to the skillet and cook for 4-5 minutes, stirring often.
Add the bell peppers, broccoli and snow peas and cook for an additional 5-7 minutes, stirring frequently until the veggies are tender but still crisp and brightly coloured.
Reduce heat to medium and add the tofu to the pan. Give the teriyaki sauce another shake or stir then pour it into the pan. Toss to combine and cook until sauce has slightly thickened, about 1-2 minutes.
Garnish with sesame seeds and serve immediately over rice, if desired.