Step 1
In a large deep skillet or Dutch oven, add the oil and swirl to coat the bottom. Lightly season the chicken thighs with salt, then place them skin side down into the cold pan. Set over medium heat and cook without disturbing until the skin turns golden, crisp, and naturally releases from the surface, about 10 minutes. Flip the chicken and sear the opposite side for 1 to 2 minutes, just until lightly browned. Transfer to a plate and set aside.
Step 2
In the same pan over medium heat, sauté the ginger and garlic until fragrant, about 1 minute. Stir in the brown sugar, soy sauce, and 1/2 cup of water, mixing until the sugar dissolves completely. Return the chicken to the skillet, positioning it skin side up in the sauce.
Step 3
Bring the mixture to a gentle boil over medium-high heat, then lower the temperature to maintain a simmer. Cover and cook until the chicken is tender and fully cooked through, reaching an internal temperature of 165°F (74°C), about 13 to 18 minutes.
Step 4
Remove the chicken from the skillet and place it on a clean plate. Increase the heat to medium-high and bring the sauce to a boil once more. Reduce to medium-low and let it simmer for 2 to 3 minutes, stirring occasionally, until it deepens in color and slightly thickens.
Step 5
In a small bowl, whisk the cornstarch with the remaining 1 Tbsp. of cold water. Gradually stir this slurry into the sauce, cooking for about 1 minute until the mixture transforms into a glossy glaze. Return the chicken to the pan and coat thoroughly in the sauce. Garnish with sliced scallions, if desired, and serve warm.