In a small bowl, whisk together the soy sauce, fish sauce, cornstarch and 1 teaspoon water, making sure there are no lumps. Stir in the hoisin, chili paste and sesame oil.
Heat a large skillet over medium heat. Add the sauce mixture, bring to a simmer and simmer until thickened, about 1 minute. Reserve the sauce in a small bowl for later.
Wipe out the skillet and heat over medium-high heat. Add the peanut oil and when hot, add the chicken and cook until just browned, 5 to 7 minutes. Remove to a plate with a slotted spoon. Add the onions, peppers and garlic and cook until just softened, about 3 minutes. Return the chicken to the skillet, stir in the sauce, tomatoes and scallions and cook for 1 minute more.
Place the cucumber noodles in a large bowl and toss with the hot chicken mixture from the skillet directly before serving. Sprinkle with torn basil and serve.