Heat olive oil over medium-high heat in a large pot or Dutch oven. Add the tofu cubes and cook until browned on all sides, stirring occasionally about 5 minutes. Remove tofu from the pot and set aside.
In the same pot, add the chopped red and green bell peppers, onion, garlic, and ginger. Cook until vegetables are tender, about 5 minutes.
In a small bowl, whisk together the cornstarch, soy sauce, hoisin sauce, rice vinegar, brown sugar, sesame oil, and red pepper flakes (if using).
Add the tofu back to the pot with the vegetables. Pour the sauce over the tofu and vegetables and stir to coat everything evenly. Add ¼ cup of water to the pot.
Bring the mixture to a simmer, then reduce heat to low and cover the pot. Let it cook for 10-15 minutes, stirring occasionally, until the sauce thickens and the tofu is heated.
Add the roasted peanuts and stir to combine.
Serve hot with brown rice and garnished with sliced green onions if desired.