Preheat the oven to 180 °C (356 °F) and grease an oven dish so that you can put the peppers in it.
Chop the onion, chop or crush the garlic and cut the mushrooms into small pieces. Heat a frying pan with oil and add the onion. Fry until the onion has turned translucent. Then add the garlic and mushrooms and let it fry for a while. Let the mixture cool.
Heat a small frying pan (without oil or butter) and roast the pine nuts in it. Do not set the heat too high, as the pine nuts can burn very quickly. So keep an eye on them! The pine nuts are ready when they are slightly discolored.
In the meantime, cut off the tops of the peppers and remove the seeds.
Put the vegetarian minced meat, egg, pine nuts, mushroom mixture, mustard, tomato paste and Italian herbs in a bowl and mix well. Add salt and pepper to taste.
Fill half of each pepper with the mixture and then top it with a layer of grated cheese. Then fill the bell pepper until it is completely full and then add another layer of grated cheese. Place the stuffed peppers in the baking dish.
When all the peppers are filled, put the dish in the oven and bake for about 25 minutes. You can also bake the top of the peppers.