Cook the Rice: Cook the whole-grain rice according to package instructions. Fluff with a fork and set aside. In a small bowl, mix together the sliced red onion, lemon juice, salt, and red pepper flakes (if desired). Toss well and let sit for at least 10 minutes to pickle.
Prepare the Tofu: Drain and press the tofu to remove excess water. Cut into cubes.
Season and Grill: In a bowl, toss the tofu with olive oil, salt, and black pepper. Heat a non-stick skillet over medium-high heat and add the tofu. Grill for 5–7 minutes, flipping to brown all sides. Set aside. In a small bowl, whisk together peanut butter, tahini, soy sauce, rice vinegar (or lime/apple cider vinegar), sugar, ginger, garlic, sriracha (or red pepper flakes), and sesame oil (if using). Adjust to taste and set aside.
Build the Bowl: Divide the cooked rice among serving bowls. Arrange the cucumber, red cabbage, carrot ribbons, mango, spinach, grilled tofu, and avocado on top.
Add Pickled Onions and Sauce: Top with pickled red onions and drizzle with peanut sauce.
Optional Toppings: Sprinkle with sesame seeds, fresh cilantro, and serve with lime wedges for squeezing over the bowl. Enjoy!