An hour before starting to cook, drain your tofu and place it on a plate between two super absorbent paper towels, and then place a medium saucepan on top of it. Next, fill the saucepan with something heavy. This will press much of the moisture out of the tofu so that it browns well.
Cube the tofu into bite-sized pieces and prep your vegetables.
In a wok or a large pan, heat 1/2 Tbsp peanut oil over medium heat for two minutes, and then add the tofu to the pan. Cook until the underside is browned (about 3 minutes), and then flip and cook the opposite side until it is also browned. Remove from the pan and set aside.
Add the remaining 1/2 Tbsp peanut oil to the pan and heat. Next, add the eggplant, green peppers, white onions, and chili pepper to the pan. Sauté until the eggplant begins to soften, but doesn't get mushy. This will take approximately 5 minutes.
While the vegetables are sautéing prep your stir fry sauce by whisking together water, tamari, low-sodium soy sauce, garlic, ginger, light brown sugar, and cornstarch in a small mixing bowl. Make sure there are no cornstarch clumps, otherwise it won't thicken correctly.
Once the eggplant has finished cooking, add the water chestnuts and the simmer sauce to the pan. Cook until the water chestnuts are heated through and the sauce is simmering and thickened.