Toss the chicken or tofu in 1 TBSP of cornstarch in a bowl or zip top bag. Set aside.
To a small bowl, add the sriracha, soy sauce, sugar, water and rice vinegar. Whisk well and taste it - adjust the sauce as needed (add more sugar to make it less spicy, add more sriracha to make it spicier) and set aside. In a separate small bowl, mix 1 TBSP water and cornstarch and set aside.
Add 1 TBSP oil to a skillet over medium heat, followed by the garlic and ginger. After a minute, add the frozen veggies with salt and pepper to taste and cook uncovered until heated through, stirring occasionally - about 7-10 minutes (depending on how long they've been out of the freezer). Then remove the veggies from the pan and set them aside.
Add 1 TBSP olive oil to the same skillet over medium heat. Then add the chicken or tofu. Add salt and pepper to taste and cook on each side for about 5-7 minutes, until the edges of the chicken/tofu look crispy. For tofu, you can flip/turn more often. For chicken, just flip once or twice. When finished, turn off the heat, but keep the chicken/tofu in the pan for now.
While the chicken/tofu cooks, cook the rice according to the package.
Last but not least, the sauce. Pour the soy sauce and sriracha mixture in a small saucepan over medium heat and cook until it reaches a gentle boil. Give the cornstarch and water a quick stir, then slowly add it to the saucepan while whisking. Cook until the sauce has thickened (about a minute) Then, turn off the heat and pour it over the cooked chicken/tofu and toss the pieces to coat them in the sauce.
Assemble: Divide the cooked rice between three meal prep containers, followed by the veggies. (I mixed my rice and veggies together). Then add the chicken/tofu on top. Garnish with sesame seeds and green onion if desired.