Bring a pot of water to a boil and salt generously.
Finely dice onion.
2. Cook chicken
Pat chicken thighs dry with paper towels, then season with salt and pepper.
Heat an ungreased frying pan over medium heat, lay in thighs and cook for 5 minutes per side, flipping from time to time, pressing gently with a spatula to develop a crisp, deep golden crust.
Transfer thighs to a warm plate and cover.
Keep frying pan over heat—do not wipe it clean.
Drop spaghetti into boiling salted water and cook for 7 minutes.
Reserve half a cup of starchy pasta water before draining.
3. Start cooking cabbage
In the same frying pan, saute onion for 1 to 2 minutes.
Add cabbage and cook for 4 to 5 minutes, stirring occasionally to let it brown and caramelize; it will reduce in volume by half.
Sprinkle in garlic powder and a pinch of salt, then stir to combine.
4. Finish cooking cabbage
Dissolve miso paste in 3 tablespoons of warm pasta water and pour into cabbage.
Stir well and simmer gently over medium-low heat for another 5 minutes.
5. Finish
Turn heat under frying pan down to lowest.
Add spaghetti, half of parmesan, and half of reserved pasta cooking water to cabbage.
Toss for about a minute until cheese and starchy water emulsify into a silky, glossy cream sauce.
6. Serve
Divide pasta between plates. Finish with freshly cracked black pepper to taste and sprinkle with remaining cheese.