Combine the rice and 4 cups of water in a saucepan and bring to a boil. Cover and reduce heat to low; simmer until rice is tender and water has been absorbed, about 20 minutes.
In a separate saucepan, whisk together the peanut butter, 1 cup hot water, soy sauce, rice vinegar, sugar, garlic, green onions and red pepper flakes. Bring to a simmer over medium heat. Cook, stirring occasionally, and adding more hot water 1/4 cup at a time to maintain a pourable consistency.
Bring about 1 inch of water to a boil in a large pot. Place the spinach and tofu into a steamer basket and set over the boiling water. Cover and steam until spinach has wilted and tofu is heated through, about 7 minutes.
To serve, place spinach and tofu over white rice and smother in peanut sauce.
