Preheat your oven to 400°F (200°C).

In a large, oven-safe pot or Dutch oven, heat a drizzle of oil over medium heat. Add the chopped onion and sauté for about 5 minutes until they become translucent.

Add the Thai red curry paste to the pot with the onions, stirring well to combine and cook for about 2 minutes.

Pour in the canned tomatoes along with the coconut milk, stirring until everything is well blended. Let this mixture simmer gently for about 3-5 minutes.

Carefully nestle the chicken thighs or drumsticks into the pot and sprinkle in salt.

Cover the pot with a lid and transfer it to your preheated oven. Bake for about 1 hour.

After the chicken has cooked through, remove the pot from the oven and stir in the fresh spinach.

Serve the curry over fluffy jasmine rice or alongside naan bread.