Whisk together all the sauce ingredients in a small bowl until the sugar dissolves. Set aside.

Dice the chicken thighs into even 1.5-inch pieces. Don't pat them dry; the natural surface moisture helps the cornstarch stick.

Place the chicken in a large bowl, add the cornstarch, and toss until evenly coated. Add 1-2 tablespoons more cornstarch if any areas remain uncoated.

Heat the oil in a large, heavy-bottomed pan over medium-high heat to 350°F. Fry the chicken in small batches for 5-7 minutes total, keeping the oil between 325-350°F. Don't move the chicken until the bottom is golden and crisp, then flip and fry until the chicken reaches 165°F. Between batches, return the oil to 350°F and remove any browned bits. Transfer to a wire rack to drain.

Remove the remaining oil from the pan, leaving 2 teaspoons. Add the chopped onions and cook for about 1 minute until crisp-tender.

Whisk the sauce again, then pour it into the pan. Simmer for 30-45 seconds until the sauce thickens enough to coat the back of a spoon.

Add the fried chicken and toss until evenly coated in the sauce. Garnish with green onions, remove from the heat, and serve immediately.