Preheat the oven to 190 C. Place the banana peppers on a baking tray. Roast for 30 mins. The skins will look puffed up and browned. Remove and set aside to cool slightly.
Sauce: Whilst they roast, make the sauce. Blanch the tomatoes in hot water until the skins start to peel away. Discard the skins. Blitz the tomatoes in a blender and set aside.
Heat the oil in a pot and sauté onions and garlic until browned. Add the pureéd tomatoes, salt, oregano, and chilli flakes. Cover and cook over a medium heat until the sauce reduces and thickens—about 10 mins. Stir occasionally while the sauce cooks.
Set aside until ready to use. The sauce can be made up to a week in advance. Or kept frozen for a month.
Once the peppers are cool enough to handle, starting at the top, peel the papery skins in a downward motion. Some will peel away completely intact whilst some may not. This is completely fine. Just try and get most of the skins off without tearing the peppers too much. (Tears can be patched up later.) Pinch away and discard the stems.
Once you have the skins off, run your finger into the top layer gently, and open them up flat (see photo in the post above). Remove the seeds and discard. Lay them out on a large surface or your kitchen counter.
Filling: Add the spinach to a dry pan and move it around for 2-3 mins until just wilted. Drain in a colander, pushing it down with a spoon to get all the water out. Add the spinach, paneer, green chillies and salt to a blender. Pulse until it comes together into a coarse paste.
Divide the paneer paste into 15 portions. Place the filling on one side of the opened banana pepper. Roll it up encasing the filling within. Repeat with the rest of the peppers.
(Note: if using torn peppers, roughly wrap them as well as you can around the filling and place them in the dish - it’s fine even if some of the filling starts to fall out).
Assembly: pour ¾ ths of the tomato sauce at the bottom of a baking dish. Place the rolled peppers on top. If your dish is not large enough, place the rolls in 2 layers.
Pour the remaining sauce over the top. Top with grated mozzarella. Bake in a preheated 180 C oven for 20-25 mins until the cheese has melted through and the sauce is bubbly. Top with chopped basil.
Serve immediately with slices of bread to mop it all up with.