Slice each chicken thigh horizontally into 2–3 thinner pieces so they cook evenly; set aside.
In a mixing bowl, whisk together oil, gochujang, gochugaru (or chili powder), sesame oil, soy sauce, sugar, minced ginger, minced garlic, and salt until smooth and the gochujang is fully dissolved.
Add the sliced chicken to the marinade and toss to coat thoroughly. Marinate for at least 30 minutes, preferably up to 2 hours.
Preheat a grill or skillet (or oven to 400°F / 207°C). Cook the chicken until slightly charred and cooked through, about 6–8 minutes per side on a hot grill or skillet, or about 20 minutes in the oven.
Transfer to a plate and serve immediately with steamed rice.