Place butter, onion, peppers, tomato puree, garlic, herbs and mushrooms in a large microwavable casserole dish.
Cover and cook for 10 minutes stirring halfway.
Stir in the rice, stock and salt.
Cover & cook for 13 minutes.
Place fillets in a shallow microwaveable dish.
Cover and cook for 7 minutes re-arranging halfway through.
Flake the fish & stir into rice mixture.
Heat through.