Heat the oil in a large pan, then add the onion. Cook very gently for about 10 minutes. Stir in the garlic and ginger and cook for a further minute.
Add the spices and cook for 1 minute, then add the butternut squash and red peppers. Cook, stirring, for about 3 minutes.
Stir in the tomatoes, chicken and stock; simmer for 25-30 minutes until the sauce has thickened.
Add the spinach to wilt, then sprinkle in the garam masala. Stir in the yogurt and cook for 1 minute.
Meanwhile, cook the basmati rice to pack instructions and serve together with the curry and the naan.