Preheat the oven to 375 F/ 190 C/ 180 C fan/ gas mark 5. Line a large baking sheet with parchment and place the halved peppers cut side up on top. Lightly brush the edges of the peppers with a little oil and bake in the centre of the oven for 20 minutes.Remove from the oven and pour the juices that will have collected inside each pepper into a bowl (do not discard them!). Leave the oven on.
In the meantime prepare the filling. In a medium sized pan heat up half the oil, add the mushrooms and cook over a fairly high heat for 5-6 minutes until all the moisture has evaporated and the mushrooms are starting to brown, stirring often. Transfer the mushrooms to a bowl.
Using the same pan heat up the remaining oil, add the onion and garlic and cook over a low/medium heat for 3 minutes until softened stirring often.
Add the meat, passata, tomato puree, balsamic vinegar, seasoning and herbs and stir until thoroughly combined. Cook over a medium/high heat for a few minutes until the meat is fully cooked, stirring often.
Add the spinach and stir until it wilts.
Add the mushrooms, juices from the peppers and continue cooking over a high heat for a further 3-4 minutes or until the mixture thickens stirring often.
Remove from the heat, stir in 2 tablespoons of the parmesan cheese, adjust the seasoning as needed and spoon the mixture into your pepper halves pressing in to remove any air pockets. Sprinkle the remaining parmesan over each pepper.
Bake in the centre of the oven for 25 minutes. Remove from the oven, garnish with fresh parsley and serve immediately.
Notes
I used large bell peppers, but if your peppers are smaller you may need to use 5.
Use any ground meat (make sure it contains around 7% fat).
Once you’ve combined all the filling ingredients (including the juices from the peppers) cook the mixture for a few minutes to remove excess moisture, though the filling should still be quite moist before stuffing the peppers (see the photos).
These no-rice stuffed peppers are best served immediately.
Refrigerate leftovers covered for up to 3 days. Reheat in the microwave.