The key to perfectly sauced noodles in this recipe is using only half the peanut sauce, plus a hefty dose of pasta cooking water to lightly coat the noodles and vegetables. The rest can go on top! The sauce will thicken as it sits, so make it so that it has a looser consistency when it’s in the skillet and it will be just right when it’s time to eat.
Bring a large pot of water to a boil. Toss the chicken with 1 tablespoon sambal oelek, 1 teaspoon soy sauce and a pinch each of salt and pepper. In a separate bowl, whisk the remaining 2 teaspoons sambal oelek and 2 tablespoons soy sauce, the peanut butter, vinegar, 3 tablespoons water, 1/4 teaspoon salt and a few grinds of pepper.
Salt the boiling water; add the spaghetti. Cook 1 to 2 minutes less than the label directs for al dente, adding the boy choy in the last 2 to 3 minutes. Reserve 1 cup cooking water, then drain.
Heat 1 tablespoon vegetable oil in a large nonstick skillet over medium-high heat. Add the chicken and cook until just cooked through, 3 to 4 minutes per side. Remove to a cutting board.
Return the skillet to medium-high heat and add the remaining 1 tablespoon vegetable oil. Add the snow peas and bell pepper; cook, stirring, until crisp-tender, 1 to 2 minutes. Add the spaghetti and bok choy and a pinch each of salt and pepper; cook until combined, 3 to 5 minutes. Add half the peanut sauce and 1/2 cup of the reserved cooking water. Toss to coat, adding more cooking water as needed. Stir in all but 2 tablespoons scallions. Season with salt.
Slice the chicken. Divide the noodles and chicken among bowls. Top with the remaining peanut sauce and reserved scallions.
If you have a peanut allergy, use smooth no-stir almond butter in place of peanut butter; add a little more water if needed.