Step 1
In a medium mixing bowl, combine the hot mustard, soy sauce, vinegar, vegetable oil, grated garlic, grated ginger, crushed red pepper, and kosher salt. Stir until all the ingredients are thoroughly blended. Add the chicken thighs to the bowl and toss them gently to ensure they are evenly coated with the marinade. Cover the bowl and refrigerate for a minimum of 20 minutes, or up to 24 hours for a deeper infusion of flavors.
Step 2
When ready to cook, remove the chicken from the refrigerator and allow it to sit at room temperature briefly. Prepare the grill by lightly oiling the grates with a paper towel dipped in vegetable oil. Heat the grill to medium-high. Once hot, place the chicken thighs directly onto the grates. Grill, covered, until they develop a lightly charred appearance, approximately 5 to 7 minutes. Flip the chicken and continue cooking on the other side until fully cooked through, around 4 to 5 minutes more.
Step 3
Transfer the cooked chicken to a serving dish. Garnish with chopped scallions or cilantro, if desired. Serve immediately alongside complementary side dishes for a well-rounded and satisfying meal.