Sprinkle chicken thighs with salt and pepper, then set aside
Heat oil over medium. Add onion and cook, stirring, until softened, about 10 minutes. Add carrots and cook until softened, about 6 minutes. Stir in garlic and cook for another minute or so
Add chicken and sake (or white wine). Cook, stirring occasionally, until meat is no longer pink on the outside, about 5 minutes
Pour in dashi (or chicken broth) - it should just cover the ingredients. Stir in tonkatsu sauce (or soy sauce) and red pepper flakes. Bring liquid to a boil and bring the heat back down to maintain a lively simmer. Stir in butter and cook until sauce's liquid is slightly thickened, 30-35 minutes
Butter a 2 quart casserole dish and heat the broiler
Stir spinach into the pan. Taste mixture and season with salt and pepper. Add rice to casserole dish, then pour chicken mixture on top of rice. Top with Parmesan, mozzarella, and panko
Broil the dish on high for at least 3 minutes, until cheese has melted and turned a golden brown (watch to make sure it doesn't burn)
Garnish with chopped parsley and serve