Combine fish sauce, brown sugar, ginger, and minced garlic in a large bowl, stirring with a whisk. Reserve 1 tablespoon mixture. Add chicken thighs to bowl; toss to coat. Refrigerate chicken 2 to 4 hours, tossing occasionally. Heat a grill pan over medium heat; coat pan with cooking spray. Add chicken, skin side down; cook 15 minutes, turning occasionally so as not to burn (you want a light char). Remove chicken to a serving platter. Drizzle reserved 1 tablespoon fish sauce mixture over chicken; sprinkle with green onions.
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