Heat a large skillet over medium high heat and add the olive oil.
Add the sliced sausage rounds and stir and cook until the sausage starts to brown.
Mix the sliced bell peppers and onion into the pan and stir and cook until the peppers become fork tender, but still have a little bite to them. This will be about 5-7 minutes.
While the peppers are cooking, lay your hoagie buns open faced on a sheet pan. Spread the top inside with butter, sprinkle a generous amount of parmesan cheese on the butter, and then sprinkle some parsley flakes on the cheese.
Put the pan with the bread in the oven and broil on the LOW setting for about 3-5 minutes, or until your bread starts to brown and then cheese melts. Check it carefully because the bread can brown quickly.
While the bread is in the oven, add your pasta sauce to your sausage and veggie mixture and stir to combine. Turn the heat off when the pasta sauce is hot.
Once your hoagie buns are out of the oven, spoon your sausage and veggie mixture onto the bottom half of each bun, then close each one to make a sandwich. Enjoy!