Combine 3 tablespoons soy sauce, 2 tablespoons lime juice, 1 tablespoon oil, minced garlic, and ground ginger in a dish.

Add chicken and turn to coat. Marinate for at least 10 minutes.

In a saucepan, combine the remaining 1 teaspoon soy sauce, 2 teaspoons lime juice, peanut butter, chicken broth, sugar, salt, and red-pepper flakes.

Pour the marinade from the chicken into the saucepan and bring just to a simmer, whisking until smooth.

Heat a grill pan over moderate heat.

Cook the chicken until browned and just done, 4 to 5 minutes per side.

Remove the chicken and let it rest for 5 minutes.

Cut crosswise into 1/4-inch slices.

Alternatively, heat the remaining 1 tablespoon oil in a heavy frying pan and cook and slice the chicken in the same way.

In a large pot of boiling, salted water, cook the spaghetti until just done, about 12 minutes.

Drain the pasta and toss with the peanut sauce, chicken, and scallions.

Top with chopped peanuts, if using.

Serve immediately.