Combine all the listed sauce ingredients in a small bowl and mix until the sugar is dissolved and the gochujang is evenly incorporated. Set aside.

Dice the chicken thighs into even 1.5-inch cubes and place them in a large mixing bowl. Do not pat the chicken dry; the surface moisture helps the cornstarch stick evenly.

Add the cornstarch to the chicken and toss until every piece is evenly coated. If any spots remain uncoated, add 1-2 tablespoons more cornstarch as needed.

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 at 325-350°F. Don't move the chicken until the bottom is golden and crisp, then flip and cook to 165°F. Between batches, return the oil to 350°F and remove any browned bits. Drain on a wire rack.

Remove most of the frying oil, leaving 1 teaspoon in the pan. Sauté the onions for 30-45 seconds. Whisk the sauce again, add it to the pan, and simmer until thick enough to coat the back of a spoon.

Reduce the heat to medium, add the fried chicken, and toss until evenly coated in the sauce. Garnish with green onions and sesame seeds, then serve immediately.