1. Drain the tofu block and tightly wrap with paper towels or a kitchen towel to remove excess liquid. Let sit for at least 30 minutes. Place on a cutting board and cut the tofu into one-inch cubes.
2. Place the cornstarch into a shallow dish (like a pie pan). Lightly toss the tofu cubes into the cornstarch to coat all sides. Heat a heavy skillet on medium high heat using 2 tablespoons of the canola oil. Add the tofu cubes and brown the tofu on all sides, about 3 to 4 minutes, until crispy. Remove the tofu from the pan and set aside.
3. In the same pan, add the remaining one tablespoon of canola oil and heat on medium-high. Add the minced garlic, onions, and bell peppers. Sauté until fragrant and onions are translucent. Remove the pan from the heat.

Make the sauce:

1. In a small mixing bowl, whisk together the soy sauce, apple cider vinegar, ketchup, honey, ginger, and red pepper flakes. In a separate small bowl, whisk together the corn starch with 2 tablespoons water.
2. Add the sauce mixture to a small saucepan on the stove at medium heat. Add the in the cornstarch mixture and keep stirring until the sauce thickens, about 30 seconds. Immediately remove from the heat.

To serve: Pour the sauce over the veggies and add in the cubed tofu, stirring to thoroughly coat with the sauce. Garnish with chopped green onions, and serve with a side of steamed brown rice if desired.