Heat a pot of salted water to boiling, then add the pasta and cook until almost tender. Reserve a cup of pasta water then drain the pasta. Preheat your oven to 180 C (350 F). Get out a 7x11 baking dish.

While the pasta is cooking, cut the tops off of the peppers, remove the stems and dice the tops. (I had an extra half a green bell pepper in the fridge so I diced that along with the tops.)

Slice the mushrooms at a 45 degree angle, a technique called Sogigiri in Japanese, this provides a larger flat cut for even cooking.

All the mushrooms sliced. I think there was 9 in the pack.

Place the peppers standing up in the baking dish. If a pepper does not stand up quite right, slice a bit off the longest lobe on the bottom so the pepper stands upright, if you sliced off part of a lobe and mad a hole in the bottom, no worries, take the piece you sliced off and put that on the inside to cover the hole. (I was going to use a 9x9 dish but that would not have enough room for the extra pasta so I switched to a 7x11 dish.)

Heat a tablespoon or two of olive oil in a large non stick pan on medium heat. When hot, add the the mushrooms.

Saute the mushrooms until the water they let out is evaporated. Remove the mushrooms to a bowl and set them aside.

Same pan, add another tablespoon of olive oil and add the diced peppers, saute until tender. Remove them to a bowl and set them aside.

Same pan, add the butter and let that full melt, then whisk in the milk and season with black pepper as desired. When the milk is at a simmer, whisk in the Parmesan cheese. When the sauce is smooth, add the drained pasta and mix into the sauce. If the sauce is too thick, add some pasta water, a tablespoon at a time and mix in. Season with black pepper as desired. Remove from heat. (Yes, I know, my method is completely backwards as to how this is supposed to be done.)

Add the mushrooms and the diced bell peppers and mix together. (The pasta at this point is actually quite good on its own.)

Using a spoon, fill each pepper with pasta, rounded at the top is perfect and add the rest of the pasta to the dish as well. Place in the oven and back for about 30-40 minutes, depending on how crisp you like the peppers.

Remove from the oven, sprinkle the Mozzarella cheese on the tops of the peppers and any additional pasta in the dish, place back in the oven for 5-10 minutes to melt the cheese.

To serve, place a pepper on a plate, slice in half and enjoy as is or with sausage. (I just happened to have Japanese sausage in the fridge so I used that.) Enjoy.