Lightly pat the chicken thighs dry and season with a pinch of salt and pepper to help the skin release moisture as it cooks.
Whisk together honey, soy sauce, minced garlic, vinegar, ketchup (if using), and ginger in a small bowl until well combined.
Place the chicken into the slow cooker. Pour the honey-garlic mixture over the top, turning the thighs to coat them evenly.
Cook on low for 6–7 hours or on high for 3–4 hours, until the chicken is tender and the flavors have melded. If you prefer a thicker glaze, you can make a quick slurry.
To thicken the sauce, mix the cornstarch with water to make a smooth slurry. Stir into the slow cooker in the last 15 minutes of cooking, then cover and continue to cook until glossy and slightly reduced.
If you used bone-in thighs, consider removing the skin before serving for a lighter option. Spoon the sauce over the chicken and garnish with finely chopped scallions or sesame seeds if you like.
Serve the chicken with steamed rice, quinoa, or a bright greens side, and drizzle additional sauce over the rice to soak up every bit of flavor.