While you're gathering ingredients, bring a large pot of salted water to a boil for the spaghetti. Mince the garlic cloves finely and grate the ginger into small, uniform pieces—this ensures even distribution of flavor throughout the sauce. Slice the green onions and set aside for garnish. Toast the sesame seeds in a dry skillet over medium heat for 2-3 minutes, stirring frequently until fragrant and golden, then transfer to a bowl. This toasting step brings out a deeper, nuttier flavor that makes a real difference in the final dish.
Pat the chicken cubes dry with paper towels—this helps them brown better and develop a golden crust. In a shallow bowl, combine the 3 tablespoons cornstarch with the salt and black pepper, then toss the chicken cubes in this mixture until evenly coated. Let the coated chicken sit for a moment while you heat your oil. I like to do this coating right before cooking so the cornstarch doesn't absorb moisture from the chicken and become clumpy.
Heat the oil in a large skillet over medium-high heat until shimmering. Working in batches if needed to avoid crowding, add the coated chicken and let it sit undisturbed for 2-3 minutes before stirring. Continue cooking for another 3-4 minutes, stirring occasionally, until the chicken is golden brown on all sides and cooked through. The browning develops flavor through the Maillard reaction, creating a delicious caramelized exterior.
While the chicken finishes cooking, combine the soy sauce, water, cider vinegar, honey, minced garlic, grated ginger, and red pepper flakes in a bowl, whisking until the honey dissolves. Once the chicken is golden and cooked through, pour this sauce over it and simmer for 4-5 minutes, stirring occasionally, to allow the flavors to meld and the chicken to absorb the sauce. In a small bowl, whisk together the 1 tablespoon cornstarch with 1 tablespoon water to create a slurry. Pour the slurry into the simmering sauce and stir constantly for 1-2 minutes until the sauce thickens into a glossy, coat-able consistency.
By now, your pasta water should be boiling; add the spaghetti and cook according to package directions until al dente. Reserve about 1/2 cup of pasta water before draining. Divide the cooked pasta among serving bowls, then ladle the teriyaki chicken and sauce generously over each portion. Toss gently to coat the noodles with the glossy sauce, adding a splash of reserved pasta water if needed to loosen the sauce slightly.
Top each bowl with a generous sprinkle of the toasted sesame seeds and a handful of the sliced green onions. The sesame seeds add a wonderful textural contrast and nutty depth, while the green onions provide a fresh, bright finish that balances the rich, savory sauce.