Place the peppers, cut side down on a baking tray. Bake in the oven at 180C for 10 minutes. Remove from the oven and allow to cool.
Heat the oil in a frying pan and cook the onion and garlic for about 10 mins or until soft. Add the mushrooms, passata, sundried tomatoes, vinegar, chilli powder and tamari. Cook for 5 mins until the sauce has thickened.
Remove from the heat and pour into a bowl. Add the walnuts and cooked quinoa. Mix well.
Spoon the filling into the peppers and then bake in the oven for 20-30 minutes until the peppers are soft. Serve with a green salad.