Combine all of the sauce ingredients in a small pan over medium-high heat. Bring to a boil and then reduce to a simmer so that the mixture continues to bubble quickly but does not boil over. Continue cooking, stirring frequently, until it cooks down and thickens. This should take between 6 and 8 minutes and should cook down to about 1/2 cup. Remove from heat and set aside.

Heat 1 Tbsp. vegetable oil in a wok or pan over medium-high heat. Add tofu to the pan and saute until tofu is golden brown, about 5 minutes. Add garlic and stir-fry just until garlic is fragrant and begins to soften, about 1 minute. Add red pepper and onion and saute until vegetables look damp and soften slightly, about 1 minute.

Pour 1/2 of the prepared sauce into the wok and stir the tofu and vegetables to coat. Add additional sauce until the bottom of the wok is covered in sauce but the vegetables are not concealed under a layer of liquid. (I find that I need to use only about 3/4 of the sauce to obtain this balance.) Continue to stir-fry for 3-4 minutes longer, allowing the sauce to bubble on the bottom of the pan and thicken around the vegetables. Add the green onions to the pan, stirring just until combined. Remove from heat and serve immediately (with or without rice on the side).