Whisk together the sauce ingredients in a small bowl. Set aside.
Start the stir-fry:
Cut the tofu into 8 slices. Cut each slice into 2 rectangles, then cut each rectangle into 2 triangles, for a total of 32. Line a large plate with a paper towel.
Heat 1 tablespoon of oil in a large skillet or wok over medium-high heat. Add the tofu and cook on both sides until lightly browned and crisp, about 5 minutes per side. Remove the tofu from the skillet and transfer it to the paper towel-lined plate to soak up any excess oil. Sprinkle with salt.
Return the skillet to medium-high heat and add the remaining tablespoon of oil. Have all of the stir-fry ingredients at your side, ready to go. Add the white parts of the onion and the ginger to the skillet and cook, stirring constantly, 1-2 minutes, or until the onion starts to soften. Quickly whisk the sauce again and pour it into the skillet, along with the pineapple, pepper, and kumquats (if using). Cook for about 2 minutes more, or until the sauce has thickened slightly. Stir in the tofu and remove from heat.
Divide the stir-fry onto 4 plates or bowls with your grain of choice. Garnish with the reserved green onions and roasted cashews, cilantro, and/or sesame seeds.
Craving more veg-friendly recipes? Shop our collection of vegetarian and vegan cookbooks, including our new Vegan Thanksgiving and Vegan Christmas cookbooks!