Heat oven to 220°C. Add 1 tbs oil to a large, oven-proof pan with a lid and heat over high heat. Season the chicken with ¾ tsp salt flakes and ½ tsp pepper, then add to the hot oil, skin-side down. Cook for 7 minutes, without turning, to brown well.
Turn the heat down to medium-high, then stir in the onion and turn over the chicken. Cook for 5 minutes or until the onion has softened and is lightly browned. Add the tomato paste, garlic and 1 tbs thyme, and cook, stirring the paste into the onions, for 2 minutes, until fragrant and everything is nicely browned.
Add 480ml boiling water, ½ tsp salt flakes and ¼ tsp pepper, then add the spaghetti, stirring to submerge and evenly distribute. Use tongs to lift the chicken pieces so they sit on top of the spaghetti, skin-side up. Bring to a simmer, cover with a lid and transfer to the oven for 30 minutes or until the liquid is absorbed.
Meanwhile, mix together the parmesan, breadcrumbs, parsley, lemon zest and remaining 1 tbs thyme in a small bowl.
Remove the pasta from the oven and reset the temperature to a high grill setting. Sprinkle the parmesan breadcrumbs over the pasta and chicken, drizzle with the remaining 1 tbs oil and return to the centre rack to grill for 3-4 minutes until nicely browned and crisp. Leave to settle for about 5 minutes before serving warm, directly from the pan.
