Stir the tomato paste, vinegar, maple syrup, soy sauce, garlic, sambal oelek, and cayenne pepper together in a small bowl.
Place a medium skillet over medium heat and coat the bottom with 1 tablespoon of oil.
Give the oil a minute to heat up, and then add the tofu in an even layer.
Cook the tofu for about 8 minutes, flipping once or twice to achieve browning on multiple sides.
Drizzle the lemon juice over the tofu, flip a few times and cook for about a minute more, until most of the liquid has evaporated.
Remove the tofu from the skillet and transfer it to a plate.
Raise the heat to high and add the remaining tablespoon of oil.
Add the bell peppers and onion. Stir-fry for about 2 minutes, until the onion begins to brown and the pepper becomes tender-crisp.
Lower the heat back to medium. Return the tofu to the skillet and stir in the sauce.
Flip everything a few times and cook for about 1 minute more, until the sauce is heated throughout.
Remove the skillet from heat.
Season with salt to taste, and sprinkle with scallions and cilantro.
Serve.