Drain tofu; cut into 6 slices width-wise. Press tofu for at least 30 minutes, preferably 1 hour.
Mix ingredients for marinade; microwave for 2 minutes or cook over low heat in a saucepan and stir until sugar is completely dissolved. Reserve approx 2/3 cup for sauce, 1/3 cup for actual marinade.
Cut pressed tofu into cubes (6 cubes per slice). Place in a glass dish or plastic zipper bag; pour the 1/3 cup of marinade over the tofu. Marinate for at least 1 hour; preferably 4-6.
Heat 2 tablespoon oil in large skillet or wok over medium/medium-high heat. Remove tofu from marinade; coat with 1/2 cup cornstarch. Shake off excess.
Make sauce: Add reserved marinade to a saucepan, along with the 1/2 cup vegetable stock/broth. Bring to a rolling boil; boil for 2 minutes to reduce a bit and concentrate the flavors. Whisk together 1 tablespoon cornstarch and 1-1/2 tablespoon cool water in a small dish until smooth. Turn sauce down to medium-high; add cornstarch slurry, whisking continuously. Continue to stir until sauce starts to thicken; reduce to low heat. Sauce should lightly coat the whisk.
Cook Tofu: Add tofu to hot pan, being careful to not overcrowd. You’ll probably need to do it in 2 batches. Resist the urge to touch your tofu cubes until they are golden brown on one side – then use tongs to flip. Once golden brown on both sides, remove from pan. Add veggies to pan (you might need to add a bit more oil) and stir-fry until bright and tender-crisp.
Combine: Return tofu to pan with veggies. Add enough sauce so that everything is lightly coated, but not drowning in sauce.
Serve over hot rice. Add extra sauce if desired!