Whisk together the garlic, ginger, chicken broth, soy sauce, gochujang, mirin, and sesame oil. Set aside.
Heat the canola oil in a cast-iron or other heavy skillet over medium-high heat. Lightly season the chicken thighs with salt and black pepper.
Place the chicken thighs in the skillet, skin-side down. Cook for 4-5 minutes or until the skin is nicely browned. Turn and cook for 2-3 minutes on the second side. Transfer to a plate. Reserve the oil/fat in the pan.
Add the onion, reduce the heat to medium-low, and cook for 5-6 minutes or until softened.
Return the chicken thighs to the skillet and pour the gochujang sauce around them. Bring to a gentle boil.
Reduce the heat to medium-low and simmer, uncovered, for 25-30 minutes, turning the chicken 2 or 3 times so it cooks evenly. Cook until the chicken is tender and registers at least 165°F in the thickest part without touching the bone.
If you prefer a thicker sauce, transfer 1/2 cup of the cooking liquid to a small bowl. Whisk in 2 teaspoons of cornstarch until smooth, then stir the mixture back into the skillet. Simmer for 2-3 minutes or until the sauce has thickened to your liking.
Serve with brown rice, garnished with sesame seeds and thinly sliced scallions.