In a small bowl, mix up a slurry with cornstarch (1.5 tbsp) and water (2 tbsp). Stir thoroughly until the cornstarch is well dissolved.
After 1.5-2 minutes of simmering the sauce and tofu, lower the heat and take the lid off the wok.
This is a great spot to adjust for color. Add more dark soy sauce (1/2 tbsp at a time) if you want to make the sauce a deeper color and add more chili sauce if you want to make the sauce redder. Gently stir in any additions to incorporate them evenly.
Then, thicken the sauce with the slurry by pouring it in slowly and gradually all around the wok. If you prefer a thinner consistency, you may not want to use all of the slurry. Stir it in gently to incorporate, again making sure to get the spatula underneath everything and scooping upwards.
Simmer the sauce for another 2 minutes.
Then, add about half of the chopped green onions (reserve the rest for final garnish at the very end) and all of the chopped sweet peppers. Add sesame oil (1 tsp) and mix it in. Turn off the heat.
If the consistency is to your liking (remember, the sauce will continue to thicken just a touch more as it cools on the table), then go ahead and plate. If you want to thin the sauce out a bit, add water, a tbsp at a time, and heat it back up for a few seconds. Then, plate and serve. Enjoy!
(If you want to serve a group of people with a range of spice preferences, provide a few servings of homemade chili oil at the table!)