Chili pepper notes: Different types of dried red chilis will vary in their spice level. If you're unsure how spicy yours are, I recommend giving one a taste. If you don't have whole dried red chilis, you can also substitute red chili flakes. (Start with 1/2 teaspoon. Add more to boost the spice, or omit altogether for a milder dish.)
Brown the tofu: Heat 1 tablespoon peanut oil (or other vegetable oil) in a nonstick wok (preferred) or skillet over medium-high heat. When the oil shimmers, carefully add the marinated tofu (it may splatter a little). Pan-fry for 3-4 minutes on the first side without touching, until golden-brown. Then flip and repeat a few more times until the tofu pieces are evenly browned. This usually takes about 10-12 minutes in total. Remove the tofu to a plate and set aside.
Stir-fry aromatics: If needed, add another 1 tablespoon of oil to the same pan. Add dried red chili peppers (if using), garlic, ginger, the white portions of the green onions, and the peanuts. Stir-fry for 2-3 minutes, until the green onions are becoming translucent.
Add veggies and tofu: Add in the chopped red bell pepper, zucchini, and water chestnuts. Stir-fry for another 3-4 minutes, or until the peppers and zucchini are as tender as you like. Then add back in the pan-fried tofu and heat for another minute or so.
Add sauce: Reduce the heat to medium. Give the sauce a quick whisk to redistribute the cornstarch, then pour it in. It should begin to bubble immediately. Stir constantly for another 1-2 minutes until the sauce thickens to your preference. If you want to thin the sauce out slightly, just stir in an extra splash of water or veggie broth. Fold in the reserved green parts of the green onion and serve.